Update after editing my PR - the bugs are:

1. WindowsPath('C:a').absolute() should return WindowsPath('C:\\d\\a') but returns WindowsPath('C:a').
This is caused by flawed logic in the parse_parts method of the _Flavour class.

2. WindowsPath('./b:a').absolute() should return WindowsPath('C:\\d\\b:a') but returns WindowsPath('b:a').
This is caused by the limited interface of parse_parts, and affects the Path.absolute, Path.expanduser and Path.__rtruediv__ methods.

3. WindowsPath('./b:a').resolve() should return WindowsPath('C:\\d\\b:a') but returns WindowsPath('b:a').
This is caused by missing logic in the resolve method and in Path.__str__

It'd be great if someone could review the PR so we can make progress with fixing the bugs.