Support newer mypy versions

[caseyduquettesc]

Fixes #70

Last version of mypy/main.py that allowed positional arguments:
https://github.com/python/mypy/blob/v0.971/mypy/main.py

Keyword arguments are required from 0.981 onwards (circle September 8, 2022):
https://github.com/python/mypy/blob/v0.981/mypy/main.py

[caseyduquettesc]

@alexeagle @thundergolfer PTAL? Not sure what the process is on requesting reviews. Apologies for the direct mention.

[alexeagle]

Thanks! Sorry I missed the notifications on this. Looks pretty safe to me, even though we don't have tests for it.

[alexeagle]

Somehow this PR did make CI go red on main, someone will have to look more when we have some funding to spend time on it.

[beeftornado]

Error looks the same as the one mentioned in #69