Checks whether `chars` is a control character.

def _is_control(self, char):
        """Checks whether `chars` is a control character."""
        # These are technically control characters but we count them as whitespace
        # characters.
        if char in ['\t', '\n', '\r']:
            return False
        cat = unicodedata.category(char)
        if cat.startswith('C'):
            return True
        return False