Remove path version check. We trust the travis tests and technique.
An initial attempt at a tempdir implementation. It fails because tempdir() / 'foo' tries to construct another tempdir instance.
This approach works, but still produces a TempDir objects instead of paths.
Added path._next_class to allow subclasses to override what class is used to create new instances.
Implemented _next_class as a class property rather than a class method.