Resolve "Fix mypy errors in models.py and complete type hints"

See merge request !134

Remove Python 3.7

See merge request !136

In particular:
- Query
- QueryTemplate

Only nose.tools was used, which can easily be replaced with one-liners and pytest.raises

See https://mypy.readthedocs.io/en/stable/installed_packages.html#creating-pep-561-compatible-packages for how to opt-in to mypy type checking in user code.

Add type hints to models.py

See merge request !67