pathlib ABCs: remove or namespace private attributes

[barneygale]

@encukou pointed out:

I see there's currently no distinction between pathlib-internal attribute names and names that are free for third-party subclasses to use.
Should there be a stronger namespace for these, like _pathlib_sep, _pathlib_globber & _pathlib_stack?
Or should _sep & co. be documented so users know to not override them?

PurePathBase and PathBase have ~10 private attributes for facilitating copying, ~3 for matching and globbing, and ~3 more for miscellaneous purposes. We should try to eliminate as many as possible, and namespace the rest (e.g. under _pathlib_)

Linked PRs

• GH-127807: pathlib ABCs: move private copying methods to dedicated class #127810
• GH-127807: pathlib ABCs: remove a few private attributes #127851
• GH-127807: pathlib ABCs: remove PathBase._unsupported_msg() #127855
• GH-127807: pathlib ABCs: remove PurePathBase._raw_paths #127883