Use PEP484 type hints

PEP484 defines mechanism for type hints in Python. This is useful for IDEs, static code analysis and type-checkers like "mypy".

In modern Python, when you write code like this:

def foo(a: str) -> str:
    return a

The resulting function object foo will have foo.__annotations__ which is a dict that looks like this: {'a': str, 'return': str}

pygobject already puts this kind of data in docstrings, so putting it in __annotations__ shouldn't be too difficult.