Speed up `posixpath.realpath()`

[barneygale]

Some optimizations to posixpath.realpath() are possible - see attached PR.

Linked PRs

• GH-114847: Speed up posixpath.realpath() #114848
• GH-114847: Raise FileNotFoundError when getcwd() returns '(unreachable)' #117481

[encukou]

(I meant to post this comment on the PR. Sorry!)