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/cross_module_interleaved_error_already_set.cpp

@@ -42,6 +42,9 @@ extern "C" PYBIND11_EXPORT PyObject *PyInit_cross_module_interleaved_error_alrea
     if (m != nullptr) {
         static_assert(sizeof(&interleaved_error_already_set) == sizeof(void *),
                       "Function pointer must have the same size as void *");
+#ifdef Py_GIL_DISABLED
+        PyUnstable_Module_SetGIL(m, Py_MOD_GIL_NOT_USED);
+#endif
         PyModule_AddObject(
             m,
             "funcaddr",