Factoring out find_registered_python_instance() from type_caster_generic::cast.

* docs: prepare for 2.6.2

* chore: pre-commit autoupdate

Bumps [jwlawson/actions-setup-cmake](https://github.com/jwlawson/actions-setup-cmake) from v1.6 to v1.7.
- [Release notes](https://github.com/jwlawson/actions-setup-cmake/releases)
- [Commits](https://github.com/jwlawson/actions-setup-cmake/compare/v1.6...821902fd0e0da89b13fd3161d358e96bf7550883

)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

* Always call PyNumber_Index when casting from Python to a C++ integral type, also pre-3.8

* Fixed on PyPy

* Simplify use of PyNumber_Index, following @rwgk's idea, and ignore warnings in >=3.8

* Reproduce mismatch between pre-3.8 and post-3.8 behavior on __index__ throwing TypeError

* Fix tests on 3.6 <= Python < 3.8

* No, I don't have an uninitialized variable

* Fix use of __index__ on Python 2

* Make types in test_int_convert more ~boring~ descriptive

* Force the builtin module key to be the correct type.

Previously it was always going to be a std::string which converted into
unicode. Python 2 appears to want module keys to be normal str types, so
this was breaking code that expected plain string types in the
builtins.keys() data structure

* Add a simple unit test to ensure all built-in keys are str

* Update the unit test so it will also run on pypy

* Run pre-commit.

Co-authored-by: Jesse Clemens <jesse.clemens@sony.com>

* style: adding clang-format as manual hook

* docs: adding a Clang-Format section. (#2803)

Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>

* style: avoid using unintialized variables

Tested with cmake --warn-unintialized -S . -B build

* refactor: use function for possibly uninit vars

This allows users to set e.g. `extra_compile_args=["-g"]` (temporarily,
for debugging purposes) and not have that get overridden by
Pybind11Extension's default `-g0`.

In order to minimize "order inversion" (not that it really matters,
though) I chose to collect all flags and add them at once (hence the
dropping of `*args`).  I also dropped flag deduplication, which seems
unneeded.

* CI: Intel icc/icpc via oneAPI

Add testing for Intel icc/icpc via the oneAPI images.
Intel oneAPI is in a late beta stage, currently shipping
oneAPI beta09 with ICC 20.2.

CI: Skip Interpreter Tests for Intel

Cannot find how to add this, neiter the package `libc6-dev` nor
`intel-oneapi-mkl-devel` help when installed to solve this:
```
-- Looking for C++ include pthread.h
-- Looking for C++ include pthread.h - not found
CMake Error at /__t/cmake/3.18.4/x64/cmake-3.18.4-Linux-x86_64/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:165 (message):
  Could NOT find Threads (missing: Threads_FOUND)
Call Stack (most recent call first):
  /__t/cmake/3.18.4/x64/cmake-3.18.4-Linux-x86_64/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:458 (_FPHSA_FAILURE_MESSAGE)
  /__t/cmake/3.18.4/x64/cmake-3.18.4-Linux-x86_64/share/cmake-3.18/Modules/FindThreads.cmake:234 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  tests/test_embed/CMakeLists.txt:...

fix: only allow integer type_caster to call __int__ method when conversion is allowed; always call __index__ (#2698)

* Only allow integer type_caster to call __int__ or __index__ method when conversion is allowed

* Remove tests for __index__ as this seems to only be used to convert to int in 3.8+

* Take both `int` and `long` types into account for Python 2

* Add test_numpy_int_convert to assert tests currently fail, even though np.intc has an __index__ method

* Also consider __index__ as noconvert to a C++ integer

* New-style classes for Python 2.7; sigh

* Add some tests on types with custom __index__ method

* Ignore some tests in Python <3.8

* Update comment about conversion from np.float32 to C++ int

* Workaround difference between CPython and PyPy's different PyIndex_Check (unnoticed because we currently don't have PyPy >= 3.8)

* Avoid ICC segfault with py::arg()

* docs: update build description slightly

* docs: missing ticks

* CI: Intel icc/icpc via oneAPI

Add testing for Intel icc/icpc via the oneAPI images.
Intel oneAPI is in a late beta stage, currently shipping
oneAPI beta09 with ICC 20.2.

* CI: Skip Interpreter Tests for Intel

Cannot find how to add this, neiter the package `libc6-dev` nor
`intel-oneapi-mkl-devel` help when installed to solve this:
```
-- Looking for C++ include pthread.h
-- Looking for C++ include pthread.h - not found
CMake Error at /__t/cmake/3.18.4/x64/cmake-3.18.4-Linux-x86_64/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:165 (message):
  Could NOT find Threads (missing: Threads_FOUND)
Call Stack (most recent call first):
  /__t/cmake/3.18.4/x64/cmake-3.18.4-Linux-x86_64/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:458 (_FPHSA_FAILURE_MESSAGE)
  /__t/cmake/3.18.4/x64/cmake-3.18.4-Linux-x86_64/share/cmake-3.18/Modules/FindThreads.cmake:234 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  tests/test_embed/CMakeLists.txt:17 (find_package)
```

* CI: libc6-dev from GCC for ICC

* CI: Run bare metal for oneAPI

* CI: Ubuntu 18.04 for oneAPI

* CI: Intel +Catch -Eigen

* CI: CMake from Apt (ICC tests)

* CI: Replace Intel Py with GCC Py

* CI: Intel w/o GCC's Eigen

* CI: ICC with verbose make

* [Debug] Find core dump

* tests: use arg{} instead of arg() for Intel

* tests: adding a few more missing {}

* fix: sync with @tobiasleibner's branch

* fix: try ubuntu 20-04

* fix: drop exit 1

* style: clang tidy fix

* style: fix missing NOLINT

* ICC: Update Compiler Name

Changed upstream with the last oneAPI release.

* ICC CI: Downgrade pytest

pytest 6 does not capture the `discard_as_unraisable` stderr and
just writes a warning with its content instead.

* Use new test pinning requirements.txt

* tests: add notes about intel, cleanup

Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>

* Adding a valgrind build on debug Python 3.9

Co-authored-by: Boris Staletic <boris.staletic@gmail.com>

* Add Valgrind suppression files

- Introduce suppression file, populate it with a first suppression taken from CPython, and fix one leak in the tests
- Suppress leak in NumPy
- More clean tests!
- Tests with names a-e passing (except for test_buffer)
- Suppress multiprocessing errors
- Merge multiprocessing suppressions into other suppression files
- Numpy seems to be spelled with a big P
- Append single entry from valgrind-misc.supp to valgrind-python.supp, and make clear valgrind-python.supp is only CPython

Co-authored-by: Boris Staletic <boris.staletic@gmail.com>

* Enable test_virtual_functions with a workaround

* Add a memcheck cmake target

- Add a memcheck cmake target
- Reformat cmake
- Appease the formatting overlords - they are angry
- Format CMake valgrind target decently

* Update CI config to new action versions

* fix: separate memcheck from pytest

* ci: cleanup

* Merge Valgrind and other deadsnakes builds

Co-authored-by: Boris Staletic <boris.staletic@gmail.com>
Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>

* Fix STATIC and SHARED flags not being detected

* fix: use classic naming for all lib types

Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>

Plug leaking function_records in cpp_function initialization in case of exceptions (found by Valgrind in #2746) (#2756)

* Plug leaking function_record objects when exceptions are thrown

* Plug leak of strdup'ed strings in function_record

* Some extra comments about the function_record ownership dance

* Clean up the function_record better, in case of exceptions

* Demonstrate some extra function_record leaks

* Change  DeleteStrings template argument to free_strings runtime argument in destruct(function_record *)

* Zero-state unique_function_record deleter object

* Clarify rvalue reference to unique_ptr parameter in initialize_generic

* Use push_back with const char * instead of emplace_back

Sometimes programmers want to control this, and while it can be changed after the fact, it's commonly set via a CMAKE_ variable; if that variable is set, we should respect that (like the CMAKE_INTERPROCEDURAL_OPTIMIZATION setting).

docs: the order of alternatives for variant types matters, and follows the same rules as overload resolution (#2784)

Found with codespell

* Fix a clang warning from [-Wshadow-field-in-constructor-modified]

warning: modifying constructor parameter 'flag' that shadows a field of 'set_flag' [-Wshadow-field-in-constructor-modified]

* Change name based on review

* Update attr.h: fix a warning found by static code analyzer

Update attr.h: fix a warning found by Visual Studio static code analyzer

Severity:Warning
Code:C6323
Description: Use of arithmetic operator on Boolean type(s).
Location:	C:\src\onnxruntime\debug\pybind11\src\pybind11\include\pybind11\attr.h:547

* Update include/pybind11/attr.h

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

* Update attr.h

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

* Disable builds for 3.10.0a4, and enable a nightly 3.10-dev build

* Fix job name

* Remove deadsnakes job for now

* Add deadsnakes jobs

* There's no deadsnakes 2.7

* Add 3.10 to versions to be discovered by legacy FindPython, and fix debug input to deadsnakes/action

* Try out branch with fix

* Update to deadsnakes/action@v2.1.1

Ignore deprecation warnings about old-style __init__/__setstate__ constructors in the tests (originally done in #2746) (#2759)

* Ignore old-style __init__ warnings

* Simplify ignoreOldStyleInitWarnings with py::exec

* Only wrap single class_::defs to ignore DeprecationWarnings about old-style __init__

* Fix leak in the test_copy_move::test_move_fallback

* Fix leaking PyMethodDef in test_class::test_implicit_conversion_life_support

* Plumb leak in test_buffer, occuring when a mutable buffer is requested for a read-only object, and enable test_buffer.py

* Fix weird return_value_policy::reference in test_stl_binders, and enable those tests

* Cleanup nodelete holder objects in test_smart_ptr, and enable those tests

Fix invalid access when reinterpret_casting a non-pybind11 PyObject* to instance* (found by Valgrind in #2746) (#2755)

* enum: Add Enum.value property

* simplify

* address review