Checks whether `chars` is a whitespace character.

def _is_whitespace(self, char):
        """Checks whether `chars` is a whitespace character."""
        # \t, \n, and \r are technically contorl characters but we treat them
        # as whitespace since they are generally considered as such.
        if char in [' ', '\t', '\n', '\r']:
            return True
        cat = unicodedata.category(char)
        if cat == 'Zs':
            return True
        return False