gh-121376: Switch from stat.st_mtime to stat.mtime_ns to achieve more accuracy

[Eclips4]

• Issue: Uses a higher granularity than seconds for .pyc file invalidation #121376

[Eclips4]

This is a PoC.
If this is a right solution, the same change should be applied to compileall/py_compile (but it would be more difficult), so failures in py_compile and compileall(and couple of other tests..)are expected.

@Zheaoli @nekopsykose can you confirm this is resolves problem on your systems?
(Note: this PR only modifies Python code, but some parts of the importlib module are frozen during the compilation process of the interpreter, so you will need to rebuild the interpreter in order to apply these changes.)

[nekopsykose]

it fixes it, yeah

[Eclips4]

Closing per #121376 (comment)