Inherit from `pathlib_abc.PathBase`

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To improve maintainability when new Python versions are released and to integrate better with upstream as well as downstream libraries, we're moving to pathlib_abc.PathBase as our base class for UPath.

This draft PR makes UPath inherit from PathBase, introduces PureUPath, and a "parser" (formerly 'flavour') based on ParserBase.

There's still quite a bit of work to be done to make all tests pass, but I'm pushing this here for people to follow along.

This PR will close #114, close #193, close #221 and close #245

[ap--]

A lot of the path parsing in PurePathBase depends on the correct behavior of ParserBase.split. I should write tests for all supported filesystems how split should behave.

[ap--]

This PR was replaced by #366