Disagreement between PureWindowsPath.relative_to() and ntpath.relpath() with rootless drives #99029
Description
On Windows, the path C: means "the current directory on the C: drive".
But pathlib's relative_to() treats it as the immediate parent of C:\. This makes sense lexographically, but it's inconsistent with everything else:
C:\Users\Barney>python
>>> import ntpath, pathlib
>>> ntpath.relpath('C:/Windows', 'C:/Users/Barney')
'..\\..\\Windows'
>>> ntpath.relpath('C:/Windows', '.')
'..\\..\\Windows'
>>> ntpath.relpath('C:/Windows', 'C:')
'..\\..\\Windows'
>>> pathlib.Path('C:/Windows').relative_to('C:/Users/Barney', walk_up=True)
WindowsPath('../../Windows')
>>> pathlib.Path('C:/Windows').relative_to('.', walk_up=True)
ValueError: One path is relative and the other is absolute.
>>> pathlib.Path('C:/Windows').relative_to('C:')
WindowsPath('/Windows') # should be ValueError, as we're mixing absolute + relative pathsThis prevents us from using relpath() from pathlib, and renders the two implementations incompatible. Booo! Also prevents us from simplifying is_relative_to() down to other == self or other in self.parents
Special cases aren't special enough to break the rules. Let's make these compatible!
Previous discussion: #84538