* Add option to force the use of the PYPY GIL scoped acquire/release logic to support nested gil access, see https://github.com/pybind/pybind11/issues/1276 and https://github.com/pytorch/pytorch/issues/83101

* Apply suggestions from code review

* Update CMakeLists.txt

* docs: update upgrade guide

* Update docs/upgrade.rst

* All bells & whistles.

* Add Reminder to common.h, so that we will not forget to purge `!WITH_THREAD` branches when dropping Python 3.6

* New sentence instead of semicolon.

* Temporarily pull in snapshot of PR #4246

* Add `test_release_acquire`

* Add more unit tests for nested gil locking

* Add test_report_builtins_internals_keys

* Very minor enhancement: sort list only after filtering.

* Revert change in docs/upgrade.rst

* Add test_multi_acquire_release_cross_module, while also forcing unique PYBIND11_INTERNALS_VERSION for cross_module_gil_utils.cpp

* Hopefully fix apparently new ICC error.

```
2022-10-28T07:57:54.5187728Z -- The CXX compiler identification is Intel 2021.7.0.20220726
...
2022-10-28T07:58:53.6758994Z icpc: remark #10441: The Intel(R) C++ Compiler Classic (ICC) is deprecated and will be removed from product release in the second half of 2023. The Intel(R) oneAPI DPC++/C++ Compiler (ICX) is the recommended compiler moving forward. Please transition to use this compiler. Use '-diag-disable=10441' to disable this message.
2022-10-28T07:58:54.5801597Z In file included from /home/runner/work/pybind11/pybind11/include/pybind11/detail/../detail/type_caster_base.h(15),
2022-10-28T07:58:54.5803794Z                  from /home/runner/work/pybind11/pybind11/include/pybind11/detail/../cast.h(15),
2022-10-28T07:58:54.5805740Z                  from /home/runner/work/pybind11/pybind11/include/pybind11/detail/../attr.h(14),
2022-10-28T07:58:54.5809556Z                  from /home/runner/work/pybind11/pybind11/include/pybind11/detail/class.h(12),
2022-10-28T07:58:54.5812154Z                  from /home/runner/work/pybind11/pybind11/include/pybind11/pybind11.h(13),
2022-10-28T07:58:54.5948523Z                  from /home/runner/work/pybind11/pybind11/tests/cross_module_gil_utils.cpp(13):
2022-10-28T07:58:54.5949009Z /home/runner/work/pybind11/pybind11/include/pybind11/detail/../detail/internals.h(177): error #2282: unrecognized GCC pragma
2022-10-28T07:58:54.5949374Z       PYBIND11_TLS_KEY_INIT(tstate)
2022-10-28T07:58:54.5949579Z       ^
2022-10-28T07:58:54.5949695Z
```

* clang-tidy fixes

* Workaround for PYPY WIN exitcode None

* Revert "Temporarily pull in snapshot of PR #4246"

This reverts commit 23ac16e859150f27fda25ca865cabcb4444e0770.

* Another workaround for PYPY WIN exitcode None

* Clean up how the tests are run "run in process" Part 1: uniformity

* Clean up how the tests are run "run in process" Part 2: use `@pytest.mark.parametrize` and clean up the naming.

* Skip some tests `#if defined(THREAD_SANITIZER)` (tested with TSAN using the Google-internal toolchain).

* Run all tests again but ignore ThreadSanitizer exitcode 66 (this is less likely to mask unrelated ThreadSanitizer issues in the future).

* bug fix: missing common.h include before using `PYBIND11_SIMPLE_GIL_MANAGEMENT`

For the tests in the github CI this does not matter, because
`PYBIND11_SIMPLE_GIL_MANAGEMENT` is always defined from the command line,
but when monkey-patching common.h locally, it matters.

* if process.exitcode is None: assert t_delta > 9.9

* More sophisiticated `_run_in_process()` implementation, clearly reporting `DEADLOCK`, additionally exercised via added `intentional_deadlock()`

* Wrap m.intentional_deadlock in a Python function, for `ForkingPickler` compatibility.

```
>       ForkingPickler(file, protocol).dump(obj)
E       TypeError: cannot pickle 'PyCapsule' object
```

Observed with all Windows builds including mingw but not PyPy, and macos-latest with Python 3.9, 3.10, 3.11 but not 3.6.

* Add link to potential solution for WOULD-BE-NICE-TO-HAVE feature.

* Add `SKIP_IF_DEADLOCK = True` option, to not pollute the CI results with expected `DEADLOCK` failures while we figure out what to do about them.

* Add COPY-PASTE-THIS: gdb ... command (to be used for debugging the detected deadlock)

* style: pre-commit fixes

* Do better than automatic pre-commit fixes.

* Add `PYBIND11_SIMPLE_GIL_MANAGEMENT` to `pytest_report_header()` (so that we can easily know when harvesting deadlock information from the CI logs).

Co-authored-by: Arnim Balzer <arnim@seechange.ai>
Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>