Add type hints

[vandalt]

Hi! This adds type hints and fixes #592.

For astropy quantities, I used Quantity without any physical type or units annotations. The reasons are:

• This caused error when using Union with non-Quantity types (see https://github.com/astropy/astropy/issues/17017)
• In retrospect, this would have been equivalent to changing the behaviour of astroplan, by introducing new unit validation that was not present before this PR, which I think would have been out of scope anyways.

The tests are all passing on my machine.

I was not sure what the formatting guidelines were for this package. Let me know if there are issues on that side!

[vandalt]

I updated formatting so that flake8 check would pass.

I also patched the typing.Self to be backward compatible for Python<3.11.

Currently the tests for Python 3.7 are failing because I used types with subscripts (e.g. np.ndarray[float]). Python 3.7 is EOL, but let me know if I should still make the type hints backward compatible.