Overwrite pidiff symlink if it already exists

[rohanpm]

If pidiff has already been linked in the .pidiff virtualenvs, but to a different instance of pidiff, a run of the tool will fail with "FileExistsError: [Errno 17] File exists".

This can occur if e.g. a project uses pidiff within tox, but a dev also directly runs pidiff.

It would be better if pidiff overwrites the symlink when it already exists.