Move `_PurePathBase` and `_PathBase` to a new `pathlib._abc` module, and
drop the underscores from the class names.
Tests are mostly left alone in this commit, but they'll be similarly split
in a subsequent commit.
The `pathlib._abc` module will be published as an independent PyPI package
(similar to how `zipfile._path` is published as `zipp`), to be refined
and stabilised prior to its possible addition to the standard library.
Barney Gale