Use PurePath directly instead of os.path.sep in rewrite.py

[nicoddemus]

Given we are already creating a PurePath, just pass the parts directly to it.

This avoids using os.path.sep, that although is an official API, seems not to be available in all systems.

Fix #9791

[nicoddemus]

The ubuntu-pypy3 is unrelated and can be ignored for now.

[The-Compiler]

FWIW this is official API: https://docs.python.org/3/library/os.html#os.sep

Also available via os.path.

I wonder, since we are changing this anyways, wouldn't something like PurePath(*parts).with_suffix(".py") be more idiomatic?

[nicoddemus]

Also available via os.path.

Oh TIL. Then I'm not sure what the problem in #9791 is. 🤔

We could use PurePath (I definitely prefer that in general) but perhaps here we should be more conservative and use os.path (which is faster AFAIK)?

EDIT: I remembered now that the source of os.path being faster was a video/post from @asottile.

[The-Compiler]

We do already use PurePath though? Also not really a fan of doing premature optimization before any profiling for this specific case.

[nicoddemus]

You folks are right, updated, thanks!

[asottile]