infer_type does not work for return type Optional[]

[Ebolon]

It seems that an optional return type does not work as expected. Extending the test case in def test_optional(tdir): results in an error:

    @cachew(tdir)
    def data() -> Optional[Job]:
        return None

Error:

...
        @cachew(tdir)
>       def data() -> Optional[Job]:
test_cachew.py:497:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
../__init__.py:694: in <lambda>
    return lambda realf: f(realf, *args, **kwargs)
../__init__.py:789: in cachew
    inferred = infer_type(func)
../__init__.py:667: in infer_type
    if not issubclass(rtype.__origin__, Iterable):
/usr/local/Cellar/python@3.9/3.9.5/Frameworks/Python.framework/Versions/3.9/lib/python3.9/typing.py:835: in __subclasscheck__
    return issubclass(cls, self.__origin__)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

cls = <class 'collections.abc.Iterable'>, subclass = typing.Union

    def __subclasscheck__(cls, subclass):
        """Override for issubclass(subclass, cls)."""
>       return _abc_subclasscheck(cls, subclass)
E       TypeError: issubclass() arg 1 must be a class