[3.13] gh-119344: Make critical section API public (GH-119353) (#120856)

This makes the following macros public as part of the non-limited C-API for
locking a single object or two objects at once.

* `Py_BEGIN_CRITICAL_SECTION(op)` / `Py_END_CRITICAL_SECTION()`
* `Py_BEGIN_CRITICAL_SECTION2(a, b)` / `Py_END_CRITICAL_SECTION2()`

The supporting functions and structs used by the macros are also exposed for
cases where C macros are not available.
(cherry picked from commit 8f17d69b7bc906e8407095317842cc0fd52cd84a)