Add mypy type checking

[dstansby]

Since we're already using black in this repository, I thought it was worth experimenting with mypy here too. This PR adds basic mypy pre-commit config, and fixes all the issues it had.

[nabobalis]

Oldest failure is:

../../.tox/py38-oldestdeps/lib/python3.8/site-packages/sunkit_image/tests/test_time_lag.py:7: in <module>
    from sunkit_image.time_lag import cross_correlation, get_lags, max_cross_correlation, time_lag
../../.tox/py38-oldestdeps/lib/python3.8/site-packages/sunkit_image/time_lag.py:167: in <module>
    def time_lag(signal_a, signal_b, time: u.s, lag_bounds: Optional[u.Quantity[u.s]] = None, **kwargs):
../../.tox/py38-oldestdeps/lib/python3.8/site-packages/astropy/units/quantity.py:422: in __class_getitem__
    warnings.warn(
E   UserWarning: Quantity annotations are valid static type annotations only if Python is v3.9+ or `typing_extensions` is installed.

[nabobalis]

I think it's worth a changelog entry.

[nabobalis]

38-oldest is failing:

../../.tox/py38-oldestdeps/lib/python3.8/site-packages/sunkit_image/tests/test_time_lag.py:7: in <module>
    from sunkit_image.time_lag import cross_correlation, get_lags, max_cross_correlation, time_lag
../../.tox/py38-oldestdeps/lib/python3.8/site-packages/sunkit_image/time_lag.py:167: in <module>
    def time_lag(signal_a, signal_b, time: u.s, lag_bounds: Optional[u.Quantity[u.s]] = None, **kwargs):
../../.tox/py38-oldestdeps/lib/python3.8/site-packages/astropy/units/quantity.py:422: in __class_getitem__
    warnings.warn(
E   UserWarning: Quantity annotations are valid static type annotations only if Python is v3.9+ or `typing_extensions` is installed.

But I think it might be time to drop 38.

[dstansby]

Boo, I'll leave this until py38 is dropped then :thu

[nabobalis]

This PR was replaced by #156, I took the commits and merged in main.