[3.13] gh-119247: Add macros to use PySequence_Fast safely in free-threaded build (GH-119315) (#119419)

Add `Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST` and
`Py_END_CRITICAL_SECTION_SEQUENCE_FAST` macros and update `str.join` to use
them. Also add a regression test that would crash reliably without this
patch.
(cherry picked from commit baf347d91643a83483bae110092750d39471e0c2)

Co-authored-by: Josh {*()} Rosenberg <26495692+MojoVampire@users.noreply.github.com>