Introduce stub classes with initial tests on what will becomesubclassable alternatives to PurePath/Path that do not haveinverted dependencies.

Replace references in PurePath and changed function in_BasePurePathTest.test_ordering_common to test missing execution paths.This is preparation for seperating the base class and factoryfunctionality in PurePath in order to fully implement SimplePath.

Introduce new base class _PurePathBase and move all of the methodsfrom PurePath into it (except for the factory __new__) for use inbuilding SimplePath / FilePath.

SimplePath provides a subclassable alternative to PurePath which isotherwise functionally equivalent -- passing all of the existingPurePath tests.

Partial implementation of FilePath which offers all of the PurePathand SimplePath functionality and passes the equivalent existing tests.

Existing is_mount method in Path was Posix only and not crossplatform even though it was in a cross-platform base class. The existingdesign relied upon being overriden later in WindowsPath. Consolidatedcode from both into new Path.is_mount() which is now cross plaformsimiliar to all of the other Path methods.

Replace hardcorded references to pathlib.Path so that the baseclass and factory functionality of Path can be seperated.

FilePath provides a subclassable alternative to Path which isotherwise functionally equivalent -- passing all of the existingPath tests.

SimplePosixPath and SimpleWindowsPath provide a subclassablealternative to PurePosixPath and PureWindowsPath which are otherwisefunctionally equivalent -- passing all of the existing PurePosixPathand PureWindowsPath tests.

Fix _BasePurePath comparison operators so that all of the newpath classes in pathlib are comparable with one another and returnresults that are consistent with the existing behavoir, i.e.Path('/') == PurePath('/').

Document and make public FilePath, SimplePath, SimplePosixPath,SimpleWindowsPath, and PathIOMixin. Also add section to pathlibdocumentation regarding the limitations of PurePath and Path withregards to subclassing and how to use these new classes as alternatives.