Loading Slakh tracks in Windows

[migperfer]

Loading tracks from Slakh in windows result in an exception.

In slakh.py there is a particular line that uses os.sep to use the OS separator, but self._track_paths["metadata"][0] returns always the linux-like path 'slakh2100_flac_redux/train/TrackXXXXX/metadata.yaml' so using os.sep fails.

I guess importing pathlib and replacing that line for self.split = str(PurePath(self._track_paths["metadata"][0])).split(os.sep)[1] should resolve the problem, but I cannot really test it since it is impossible to clone mirdata in windows as stated in issue #517 .

[harshpalan]

Fixed with #567