refactor(mypy): Improve HSMixin type hints
Inside HSMixin: - when type checking, define `get_type` within HSMixin - remove now useless type:ignore comment inside `get_description` method Inside nodes.Constant the get_type could return a str or None: - make sure `get_type` always returns a str - change the default value of _uidtype from None to empty string - type self.uidtype as a str