Change Path.makedirs() -> Path.mkdir(parents=True)

[pradyunsg]

Toward #6050.

[xavfernandez]

needs exist_ok parameter instead of assuming it is OK

This only needs to be added to tests that would fail on directly switching this behavior.

And, in those cases, I feel we should blanket-replace path.mkdir with assert path.exists() anyway.

Since the goal is to replace tests.lib.path by Pathlib (or equivalent), I think it should behave identically. And since exist_ok defaults to False, mkdir should raise and not return self...

[pradyunsg]

I want us to tackle that in a follow up on that. We can make that change too, bringing it in line with pathlib's Path.

Right now, this PR is scoped to just getting rid of makedirs though. 🙃

[xavfernandez]

If an other PR adds exist_ok, this one is fine 🙂

[chrahunt]

Adding exist_ok is tracked in the note for mkdir here.