gh-111569: Implement Python critical section API (gh-111571)

Critical sections are helpers to replace the global interpreter lock
with finer grained locking.  They provide similar guarantees to the GIL
and avoid the deadlock risk that plain locking involves.  Critical
sections are implicitly ended whenever the GIL would be released.  They
are resumed when the GIL would be acquired.  Nested critical sections
behave as if the sections were interleaved.