Add type checking job

[effigies]

I've realized we're putting a lot of type annotations into this package but haven't been testing them.

We should add a new dependency group types or typecheck or similar, and at least use mypy. Then we should add a new tox job that runs mypy on src and tests. We may need to exclude the test data directory, since any code found in submodules should not be considered. setup.py should also be excluded.

[effigies]

Example: https://github.com/nipreps/acres/blob/f1b8d8e679879c544e647fbe06fe329514f2edfe/tox.ini#L89-L97