A PyThreadState can be in one of many states in its lifecycle, represented by some status value. Those statuses haven't been particularly clear, so we're addressing that here. Specifically:

• made the distinct lifecycle statuses clear on PyThreadState
• identified expectations of how various lifecycle-related functions relate to status
• noted the various places where those expectations don't match the actual behavior

At some point we'll need to address the mismatches.

(This change also includes some cleanup.)

• Issue: [subinterpreters] Make the PyGILState API compatible with subinterpreters #59956