Use PEP-484 type hints instead of docstring types

[goodmami]

The napoleon extension to Sphinx (which we use for documentation) accepts PEP-484 type hints as well as types in the docstrings. E.g., these are the same:

def foo(a):
    """
    foo the a

    Args:
        a (int): a bar
    """
    return a * 2

def foo(a: int):
    """
    foo the a

    Args:
        a: a bar
    """
    return a * 2

The type hints in the latter can also be used by static analysis tools for type checking, and maybe other things in the future. We should aim to (maybe gradually) incorporate type hints wherever applicable throughout the PyDelphin codebase.

[goodmami]

It seems the Napoleon support for type hints is not great. There's a third party module: https://github.com/agronholm/sphinx-autodoc-typehints

But the rendering, while informative, isn't so pretty.

Nevertheless, the type hints are probably a good way to go, but we might have to customize some CSS to make it more legible.

[goodmami]

This will be a huge effort that I'm not prepared to do right now, so I'm removing it from 1.7.0.