gh-116738: Make pwd module thread-safe

[yoney]

Make the pwd module functions getpwuid(), getpwnam(), and getpwall() thread-safe. These changes apply to scenarios where the GIL is disabled or in subinterpreter use cases.

• PR removes Py_LIMITED_API to use PyMutex, necessary for both FT-Python and subinterpreter use cases, similar to changes in the grp module (gh-126316: Make grp.getgrall() thread-safe: add a mutex #127055).

cc: @mpage @colesbury @Yhg1s

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

[yoney]

Note: I am not sure if there is a CI setup where getpwuid_r()/getpwnam_r() are not available. I tested those manually on my local machine.

[mpage]

Looks great! I'll wait to merge for a day or two to give other folks a chance to look.