bpo-44136: remove pathlib._Flavour

[barneygale]

For bpo-24132, we'd like subclasses of AbstractPath to be able to customize flavour behaviour (like as_uri()) without needing to promote _Flavour to a public class.

(Copy-pasted bug description:)

Following #18841, #25264 and #25701, pathlib._Flavour and its subclasses are looking a bit pointless.

The implementations of is_reserved() and make_uri() (~as_uri()) can be readily moved to into PurePosixPath and PureWindowsPath, which removes some indirection. This follows the pattern of OS-specific stuff in PosixPath and WindowsPath.

The remaining methods, such as splitroot(), can be pulled into Pure*Path with an underscore prefix.

I'm generally a believer in composition over inheritance, but in this case _Flavour seems too small and too similar to PurePath to separate out into 3 extra classes.

There should be no impact on public APIs or performance.

I've tried to split the changes up across commits; it may be easier to browse the diff that way.

https://bugs.python.org/issue44136