gh-116738: Make mmap module thread-safe

[yoney]

These changes make the mmap module thread-safe for FT-Python by protecting the mmap_object against race conditions and undefined behavior. The goal is to provide behavior similar to standard Python builds with the GIL enabled, rather than making mmap fully deterministic or generally recommended for multithreaded use.

cc: @mpage @colesbury @Yhg1s

• Issue: Audit all built-in modules for thread safety #116738

[ZeroIntensity]

I think that it would be helpful in the long run to switch mmap to AC rather than manually adding lock_held functions everywhere.

[corona10]

Same question from my side:
Why don t we switch to use AC with '@critical_section'?

[yoney]

Same question from my side: Why don t we switch to use AC with '@critical_section'?

@ZeroIntensity, @corona10: Thank you both for the reviews, I agree that using AC with @critical_section would be a better.

I wanted to ask for your preference: would you like to implement the switch in this PR, or should I hold off on this one and do the implementation separately, then continue with this PR after that? Alternatively, the switch could be done as a follow-up PR.

Thanks!

[ZeroIntensity]

Let's do AC in this PR.