Add tests to verify previously untested black-box class properties ofPurePath and Path in pathlib. Collectively, these tests verify all theways the classes represent themselves via isinstance, issubclass,__repr__(), etc. Moreover all class instances spawned by PurePath orPath are also similarly tested.

Add expected failing tests to pathlib that will verify the abilityof PurePath and Path to be subclassed by users. As part of this,test all of the black-box class properties of the newly subclassedPath-like class instances, and the instances generated by their methods.

Existing is_mount method in Path was Posix only and not cross-platform 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 to facilitate makingPath extensible.

Replace factory functionality of PurePath and Path with a psuedo-factorywhich appears to generate it's derived subclasses, but instead justmasquerades the new instance as one of the subclasses. Doing so makesthe resultant versions of PurePath and Path directly subclassable and assuch allows them to pass all of the previously failing subclass tests.

Add tests mirroring existing comparison tests for PurePosixPathand PureWindowsPath but extending them to direct subclasses of PurePath.

Add documentation highlighting the new direct subclass functionality ofPurePath and Path. Also add new is_posix & is_windows doctest conditionsso that system specific doctest code can still be tested instead of justbeing taken as a literal block and always skipped.