Fix minor bug in `_add_lflags` that allows a flag to be added more than once.

* docs: some final 2.6.0 updates

* docs: better warning text

Closes #2571; this is defined to share, and unlike DATADIR, is not sometimes overwritten incorrectly for storing CMake files.

* Throw error_already_set in py::len on failing PyObject_Length

* Fix tests to mach error message on PyPy

Shorten PYBIND11_EMBEDDED_MODULE macro implementation by using PYBIND11_CATCH_INIT_EXCEPTIONS (#2579)

* fix: Intel 18+ fully supported

* fix: Intel compiler workaround no longer needed

Followup on #94 now that Intel 18+ is required.

* add support for LLVM compiler

* Update include/pybind11/pybind11.h

__forced_unwind libc++ vs libstdc++

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

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

* Update PGI to 20.9

Update PGI to 20.9

* Update ci.yml

* Update ci.yml

* Update README.rst

* Update ci.yml

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

* docs: FAQ CMake updates

* docs: limitations

* ci: don't over label

* docs: update CHANGELOG, add a bit more structure

* ci: label PRs with more labels, and sooner

* docs: updates from @rwgk

* docs: address @YannickJadoul's points

* Deprecated public constructors of module

* Turn documentation comment of module_::add_object into valid doxygen documentation

* Move definition of PYBIND11_DETAIL_MODULE_STATIC_DEF and PYBIND11_DETAIL_MODULE_CREATE macros up

* Move detail::create_top_level_module to module_::create_extension_module, and unify Python 2 and 3 signature again

* Throw error_already_set if module creation fails in module_::create_extension_module

* Mention module_::create_extension_module in deprecation warning message of module_::module_

* Check scope's __dict__ instead of using hasattr when registering classes and exceptions, to allow registering the same name in a derived class scope

* Extend test_base_and_derived_nested_scope test

* Add tests on error being thrown registering duplicate classes

* Circumvent bug with combination of test_class.py::test_register_duplicate_class and test_factory_constructors.py::test_init_factory_alias

* tests: Don't run tests that often segfault

* tests: drop all cross module gil tests

* tests: try skipping all macOS Python 3.9 tests

* tests: drop macOS Python 3.9

* ci: skip cpptest on Win Py 3.8+

* docs: minor typo caught by @rwgk

* docs: minor cleanup

* ci: fix add-path command

* docs: add example of use in-place

* Update .github/workflows/ci.yml

* fix: PYBIND11_OBJECT could only be used inside the pybind11 namespace (regression)

* docs: add changelog for conversion protection change

* ci: update to Python 3.9

* feat: add a priority overload with py::prepend

* doc: fix wording as suggested by rwgk

* feat: add get_pointer

* refactor: is_prepended -> prepend (internal)

* docs: suggestion from @wjakob

* tests: add test covering get_pointer/set_pointer

Fail on passing py::object with wrong Python type to py::object subclass using PYBIND11_OBJECT macro (#2349)

* Fail on passing py::object with wrong Python type to py::object subclass using PYBIND11_OBJECT macro

* Split off test_non_converting_constructors from test_constructors

* Fix test_as_type, as py::type constructor now throws an error itself if the argument is not a type

* Replace tp_name access by pybind11::detail::get_fully_qualified_tp_name

* Move forward-declaration of get_fully_qualified_tp_name to detail/common.h

* Don't add the builtins module name in get_fully_qualified_tp_name for PyPy

* Add PYBIND11_BUILTINS_MODULE macro, and use it in get_fully_qualified_tp_name

* fix: find_import didn't work properly for classic tools

* ci: fix all files not being checked in style run

* Remove code inside 'PYPY_VERSION_NUM < 0x06000000' preprocessor if branch

* fix: more cleanup

* Remove more references to PyPy 5.7 and 5.9 in the docs

* Update comment on PyUnicode_UTF* in PyPy

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

* tests: New test for ctypes buffers (pybind#2502)

* fix: fix buffer_info segfault on views with no stride (pybind11#2502)

* Explicit conversions in buffer_info to make clang happy (pybind#2502)

* Another explicit cast in buffer_info constructor for clang (pybind#2502)

* Simpler implementation of buffer_info constructor from Py_buffer.

* Move test_ctypes_buffer into test_buffers

* Comment on why view->strides may be NULL (and fix some whitespace)

* Use c_strides() instead of zero when view->strides is NULL.

c_strides and f_strides are moved from numpy.h (py::array)
to buffer_info.h (py::detail) so they can be used from the
buffer_info Py_buffer constructor.

* Increase ctypes buffer test coverage in test_buffers.

* Split ctypes tests and skip one which is broken in PyPy2.

* WIP: module -> module_ without typedef

* refactor: allow py::module to work again

* feat: import check as a common function

* docs: add cmake to docs