GeezerVison

One of only three pieces of functionality that's available in the
object-oriented pathlib library, but not the traditional os/os.path
modules.

By moving this functionality to low-level modules, we make progress towards
being able to remove `pathlib._Flavour` in bpo-44136 and ultimately unlock
pathlib's OOPy potential in bpo-24132.

This commit also adjusts `urllib.request.pathname2url()` to use the new
implementation on Windows. On other platforms, the existing implementation
is retained due to its simplicity and its highly questionable support for
relative file URIs (like `'file:photos/foo.txt'`)