Get the py files which have been changed since rev.

def get_filenames_diff(self, r):
        """Get the py files which have been changed since rev."""
        cmd = self.filenames_diff_cmd(r)

        diff_files = shell_out_ignore_exitcode(cmd, cwd=self.root)
        diff_files = self.parse_diff_filenames(diff_files)

        return set(f for f in diff_files if f.endswith('.py'))