Add check if `str(handle)` correctly converted the object, and throw py::error_already_set if not (bis) (#2477)

* Add check if `str(handle)` correctly converted the object, and throw py::error_already_set if not

* Fix tests on Python 3

* Apply @rwgk's fixes to cherry-picked commits from #2392

The variables PYBIND11_HAS_OPTIONAL, PYBIND11_HAS_EXP_OPTIONAL, PYBIND11_HAS_VARIANT,
__clang__, __APPLE__ were not checked for defined in a minortity of instances.

If the project using pybind11 sets -Wundef, the warnings will show.

The test build is also modified to catch the problem.

* fix: support nvcc and test

* fixup! fix: support nvcc and test

* docs: mention what compilers fail

* fix: much simpler logic

* refactor: slightly faster / clearer

* Add py::object casting example to embedding docs

* Move implicit cast example to object.rst

* Move to bottom and improve implicit casting text

* Fix xref

* Improve wording as per @bstaletic's suggestion

* ci: only annotate linux for now

* style: block some common mistakes

* Add note about specifying custom base.

* Update exception docs based on PR feedback.

* Fix trailing whitespace.

Co-authored-by: Michal Sustr <michal.sustr@aic.fel.cvut.cz>

* fix: use classic extension handling unless otherwise requested

* fix: variable must be cached to be used externally

* feat: py::pos_only

* fix: review points from @YannickJadoul

* fix: review points from @bstaletic

* refactor: kwonly -> kw_only

* doc: avoid C++ types in docstrings

* A bit of rewording

* Another bit of rewording

* Third rewording

* ci: harden chrono test, mark another macos 4.9 dev failure

This should help with a little of the flakiness seen with the timing test

* Update tests/test_chrono.py

* Can also fail

Adding missing `bytes` type to `test_constructors()`, to exercise the code change.

The changes in the PR were cherry-picked from PR #2409 (with a very minor
modification in test_pytypes.py related to flake8). Via PR #2409, these
changes were extensively tested in the Google environment, as summarized here:
https://docs.google.com/document/d/1TPL-J__mph_yHa1quDvsO12E_F5OZnvBaZlW9IIrz8M/
The changes in this PR did not cause an issues at all.

Note that `test_constructors()` before this PR passes for Python 2 only
because `pybind11::str` can hold `PyUnicodeObject` or `PyBytesObject`. As a
side-effect of this PR, `test_constructors()` no longer relies on this
permissive `pybind11::str` behavior. However, the permissive behavior is still
exercised/exposed via the existing `test_pybind11_str_raw_str()`.

The test code change is designed to enable easy removal later, when Python 2
support is dropped.

For completeness: confusingly, the non-test code changes travelled through PR

Example `ambiguous conversion` error fixed by this PR:
```
pybind11/tests/test_pytypes.cpp:214:23: error: ambiguous conversion for functional-style cast from 'pybind11::detail::item_accessor' (aka 'accessor<accessor_policies::generic_item>') to 'py::bytes'
            "bytes"_a=py::bytes(d["bytes"]),
                      ^~~~~~~~~~~~~~~~~~~~
pybind11/include/pybind11/detail/../pytypes.h:957:21: note: candidate constructor
    PYBIND11_OBJECT(bytes, object, PYBIND11_BYTES_CHECK)
                    ^
pybind11/include/pybind11/detail/../pytypes.h:957:21: note: candidate constructor
pybind11/include/pybind11/detail/../pytypes.h:987:15: note: candidate constructor
inline bytes::bytes(const pybind11::str &s) {
              ^
1 error generated.
```

Fix bug roundtripping datetime.time objects after midnight in eastern hemisphere timezones (#2417) (#2438)

* Fix bug roundtripping datetime.time objects after midnight in eastern hemisphere timezones (#2417)

* tests: check more timezones

* Fix review remarks: remove useless comment and skip setting TZ environment variable on Windows

* fix: respect PYTHON_VERSION if set in classic mode

* fix: add warning when using PYTHON_VERSION

tests: fix CI by including <algorithm> to stop MSVC from complaining about std::count_if in tests/test_sequences_and_iterators.cpp (#2435)

Throw exception on returning a unique_ptr or shared_ptr nullptr (or any other holder type) from py::init, rather than crashing (#2430)

* Add note that VS2017 requires /permissive- to build in C++17 mode

* ci: test C++17 on MSVC 2017

* ci: args1/2, use args to override max cxx

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

Adapt code example in advanced/classes.rst to new handling of forgetting to call the superclass __init__ (#2429)

The main change is to treat error_already_set as a separate category
of exception that arises in different circumstances and needs to be
handled differently. The asymmetry between Python and C++ exceptions
is further emphasized.

* fix: reduce target collision in add_submodule mode

Closes #2420

* fix: update CMakeLists.txt

CPython configured with `--with-pydebug` could not use `pybind11_add_module`

* docs: move CONTRIBUTING

* docs: clarify PyPy promise

* tests: keep source dir clean

* ci: make first build inplace

* ci: drop dev setting (wasn't doing anything)

* tests: warn if source directory is dirty

* feat: FindPython support

* refactor: rename to PYBIND11_FINDPYTHON

* docs: Caps fixes

* feat: NOPYTHON mode

* test: check simple call

* docs: add changelog/upgrade guide

* feat: Support Python3 and Python2

* refactor: Use targets in tests

* fix: support CMake 3.4+

* feat: classic search also finds virtual environments

* docs: some updates from @wjakob's review

* fix: wrong name for QUIET mode variable, reported by @skoslowski

* refactor: cleaner output messaging

* fix: support debug Python's in FindPython mode too

* fixup! refactor: cleaner output messaging

* fix: missing pybind11_FOUND and pybind11_INCLUDE_DIR restored to subdir mode

* fix: nicer reporting of Python / PyPy

* fix: out-of-order variable fix

* docs: minor last-minute cleanup

* fix: throwing repr caused a segfault

* fixup! ci: include Python 3.9 RC1 (#2387)

* docs: move helpers to .github where allowed

* docs: more guidelines in CONTRIBUTING

* chore: update issue templates

* fix: review from @bstaletic

* refactor: a few points from @rwgk

* docs: more touchup, review changes