Fix a regression in handling processes that fork

[godlygeek]

We switched from pthread_atfork handlers to os.register_at_fork ones
in order to support Python 3.13, where a lock that our child fork
handler needs to acquire will not have been reinitialized in the child
process at the time when pthread_atfork handlers run but will be
reinitialized by the time os.register_at_fork handlers run.

Unfortunately, this means that if a process forks without calling the
before handler registered with os.register_at_fork (as happens when
using the "spawn" start method for multiprocessing, the default on
macOS), then our hooks are unexpectedly still active from the time the
process forks until the time that it execs another process image.

We can work around this by using a mix of both pthread_atfork and
os.register_at_fork handlers: we suspend tracking in
a pthread_atfork prepare handler, resume it in a pthread_atfork
handler in the parent process, and (if --follow-fork mode is used) we
reinitialize it in the child process from an os.register_at_fork child
handler, after the interpreter's locks have been reinitialized.

Closes: #642

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.06%. Comparing base (41248ed) to head (41ce62f).
Report is 103 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #645      +/-   ##
==========================================
+ Coverage   92.55%   93.06%   +0.51%     
==========================================
  Files          91       94       +3     
  Lines       11304    11415     +111     
  Branches     1581     2092     +511     
==========================================
+ Hits        10462    10623     +161     
+ Misses        837      792      -45     
+ Partials        5        0       -5     

Flag	Coverage Δ
cpp	93.06% <100.00%> (+7.12%)	⬆️
python_and_cython	93.06% <100.00%> (-2.66%)	⬇️

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