Add type hints and the py.typed stub

[fabiommendes]

Problem

Model utils does not include type hints and py.typed. This prevents tools such as mypy to work properly with it.

Solution

Add model_utils/py.typed and a few typing hints.

Commandments

• Write PEP8 compliant code.
• [_] Cover it with tests.
• [_] Update CHANGES.rst file to describe the changes, and quote according issue with GH-<issue_number>.
• Pay attention to backward compatibility, or if it breaks it, explain why.
• [_] Update documentation (if relevant).

[foarsitter]

Typing will be covered in #558 so I have to close this PR since it is incomplete.

[foarsitter]

See #603 for the ongoing progress.