Test typing with mypy in CI

[MattijsKneppers]

Flake8 is generally used by Ableton for python type checks

[ala-ableton]

AFAIK, flake8 doesn't do type checking.

[MattijsKneppers]

Wait, you're right of course, flake8 is only used for style checking! I misread the Python guide article in Ableton's software engineering practices. Never mind then @jby-ableton , seems like we can stick to mypy.

[MattijsKneppers]

I rephrased the title