fix: support free-threaded CPython with GIL disabled (#5148)

* Support free-threaded CPython (PEP 703)

Some additional locking is added in the free-threaded build when
`Py_GIL_DISABLED` is defined:

- Most accesses to internals are protected by a single mutex
- The registered_instances uses a striped lock to improve concurrency

Pybind11 modules can indicate support for running with the GIL disabled
by calling `set_gil_not_used()`.

* refactor: use PYBIND11_MODULE (#11)

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* Address code review

* Suppress MSVC warning

* Changes from review

* style: pre-commit fixes

* `py::mod_gil_not_used()` suggestion.

* Update include/pybind11/pybind11.h

---------

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>
Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>

tests/env.py

@@ -1,5 +1,6 @@
 import platform
 import sys
+import sysconfig
 
 import pytest
 
@@ -9,6 +10,7 @@ WIN = sys.platform.startswith("win32") or sys.platform.startswith("cygwin")
 
 CPYTHON = platform.python_implementation() == "CPython"
 PYPY = platform.python_implementation() == "PyPy"
+PY_GIL_DISABLED = bool(sysconfig.get_config_var("Py_GIL_DISABLED"))
 
 
 def deprecated_call():