Annotate code with type hints

Type hints help catch certain errors, document the code base, help static analyzers (like [mypy](http://mypy-lang.org/) or Google's [pytype](https://github.com/google/pytype)) and linters in IDEs, and let us build and maintain a cleaner architecture overall.

Annotations can be done iteratively over multiple, small Pull Requests, based on the docstrings already in place. Type hints can be tested with local installations of mypi or pytype until these static analyzers are integrated in the CI pipelines.

See https://docs.python.org/3/library/typing.html for more details.