speed up cache by approximately 42x by avoiding pathlib

[asottile]

resolves #1950

[cooperlees]

Thanks - Crazy increase. Expensive object this pathlib :D.

(will let some others look before merging)

[JelleZijlstra]

Looks like Travis is just never finishing, do we still need it?

[JelleZijlstra]

And thanks for catching and fixing this!

[ichard26]

Looks like Travis is just never finishing, do we still need it?

Travis CI for Open Source is shutting down in a few weeks so the queue for jobs is insane due to lower resources. I'm 99.99% sure we don't need it as our Test, Lint, Docs, Upload / Package, Primer, and Fuzz workflows are all on GitHub Actions. So even though we can migrate to the .com version with its 1000 free Linux minutes(?), I don't think we need to.

(more info here, here, and also here)

[ichard26]

This might need a change log entry, but other than that LGTM, thanks!

edit: maybe when cache operations are done per run rather than per file in another PR, we can add the changelog entry along the lines of "Caching overhead when formatting large quantities of files has been significantly reduced" that covers both PRs including this one

edit 2: I can't read emails apparently, Black's caching DOES NOT behave how I described it as above ... today isn't a good day for me lol

[asottile]

added the changelog entry