Add Type Checking To This Project And Add py.typed

[rmorshea]

Current Situation

Right now, we don't do any type checking with MyPy. We also do not distribute a py.typed marker file which would allow MyPy to work with django_idom when it's installed in other projects.

Proposed Actions

• Add type checking with mypy. Preferably we'd eventually check in --strict mode, but we don't have to start there.
• Distribute a py.typed marker file.

[Archmonger]

What's the benefit of MyPy over stdlib type hints?

[rmorshea]

MyPy is just the type checker. All the type hints will stay the same.