MAINT: Cleanup compatibility code for pathlib

[eric-wieser]

Path can now never be none, and PurePath is not used and not in __all__.

[BvB93]

This may be beyond the scope of this PR, but do we even need the is_pathlib_path function?

The only place where it seems to be used is in memmap.__new__ and replacing it with an
unconditional call to either os.fspath or os.fsdecode will also take care of any pathlib.Path instances.

[eric-wieser]

No we don't - but we probably can't remove things from this module without deprecating them first.

We should certainly change all the callers of these useless aliases.

[BvB93]

No we don't - but we probably can't remove things from this module without deprecating them first.

Isn't compat a private module though?

[eric-wieser]

I don't think we ever made it clear what is and is not public - it doesn't have any underscores in it, so the default user may assume it public, even if that was never our intent. It's a thorny enough issue to definitely be out of scope for this PR.

The C compat header has similar issues.

[BvB93]

Fair enough, let's keep this PR as it is then.

[charris]

Thanks Eric.