Clean up dependencies

[braingram]

Clean up dependencies:

• increase minimum versions for:
  • asdf-coordinates-schemas: to pick up latest schemas and remove version checks
  • asdf-standard: to pick up latest schemas and remove version checks
  • astropy: to pick a version ~2 years old and remove version checks
• remove pytest-astropy (unused as we have no doctest in our rst files)
• remove indirect docs requirements (managed by our dependencies, no need to list them here):
  • docutils
  • graphviz
  • matplotlib
  • sphinx-automodapi

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.06%. Comparing base (0b8f857) to head (8d15a69).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #333      +/-   ##
==========================================
- Coverage   99.06%   99.06%   -0.01%     
==========================================
  Files          71       71              
  Lines        2572     2556      -16     
==========================================
- Hits         2548     2532      -16     
  Misses         24       24              

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