Clear the cache.

def clear(self):
        """
        Clear the cache.
        """
        not_removed = []
        for fn in os.listdir(self.base):
            fn = os.path.join(self.base, fn)
            try:
                if os.path.islink(fn) or os.path.isfile(fn):
                    os.remove(fn)
                elif os.path.isdir(fn):
                    shutil.rmtree(fn)
            except Exception:
                not_removed.append(fn)
        return not_removed