Add mypy type annotations to BAC0

[gnmerritt]

BAC0 doesn't currently have many type annotations in place, I'd like to take a quick first pass at generating and adding them so that it's easier to work with the codebase.

I've previously had good luck with https://github.com/Instagram/MonkeyType and then manually editing its results where necessary.

I'd also like to add mypy to the github test workflows, so we can prevent regressions once the annotations are in place.

This is a larger change so wanted to start a discussion before writing a whole mess of code - what do you think?

[ChristianTremblay]

I'm not against... there will be some challenges but why not...

[gnmerritt]

Ok cool. Definitely won't be 100% coverage and I won't add any coverage-based checks to the workflows. I'm hoping to get a chance to work on this in the next couple weeks...

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[gnmerritt]

Finally got started on this last week! Made some good progress but still need to figure out how best to collect types for the TaskManager threads (since monkey type only runs in the parent process)

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