Progress report:

I've been working on tidying up the pathlib internals over the 3.9 and 3.10 releases. We're now in a position where:

- `pathlib._Flavour` is entirely pure, and doesn't make any `os` calls
- `pathlib._Accessor` handles all `os` access.

The internal abstractions are now much tighter, which allows us to begin refactoring them with confidence!

The next step is to remove accessors, in bpo-43012.

After that I'll finally be in a position to start working on this bug!