[issue20002] Cleanup and microoptimize pathlib

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

> Do you have performance numbers that show a significant improvement?

No. The benefit is too small (but theoretically it should be). Some changes 
also simplify the code. This is why this patch should be applied now, while 
pathlib code is not frozen yet.

For example "drv or root" replaced by "root or drv" because drv is always 
false on Unix and both arguments of "or" should be evaluated. This is very 
tiny optimization, but it has zero cos if apply it right now.

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