Add `PYBIND11_SIMPLE_GIL_MANAGEMENT` option (cmake, C++ define) (#4216)

* 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 appar...