Handle changed ntpath.isabs behaviour in Python 3.13 (#110)

[AdamWill]

ntpath.isabs no longer considers paths that start with "/" to be absolute. This is correct behaviour on Windows but causes us problems, because our PathConstants use paths that start with / and expect these to always be considered absolute. To deal with this, let's add an extra slash to the PathConstants; paths starting // are considered absolute by both ntpath and posixpath.

This is an alternative to #111 , it probably doesn't make sense to do both.