Revisit test matrix: remove python 3.7

[wtgee]

          You should probably drop Python 3.7. It is picking up a very old `astropy` (4.3.1).

Originally posted by @pllim in https://github.com/astropy/astroplan/issues/554#issuecomment-1572975888

EOL for Python 3.7 is 2023-06-27 (schedule) so we should consider removing support from our test mtarix.

The tox.ini file also specifies a number of versions of astropy that are quite old and doesn't specifically support the 5.x branch, so we should clean that up as well.