Make class variables immutable

[WilliamJamieson]

An issue pointed out in #183 is that we have many of the classes which define mutable class variables. In general, mutable class variables should not be used because the state of that variable is shared across all instances. Thus they should be made to be immutable objects.

This PR corrects this issue for this package.

Note that this issue occurs mostly for the tags and types attributes of the converters. These were turned into properties instead of tuples because ASDF does not support these values being anything other than lists (ASDF will literally segfault).

[codecov]

Codecov Report

Merging #187 (c6608be) into main (70e27a2) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #187      +/-   ##
==========================================
+ Coverage   98.63%   98.71%   +0.08%     
==========================================
  Files          55       55              
  Lines        2047     2180     +133     
==========================================
+ Hits         2019     2152     +133     
  Misses         28       28              

Impacted Files	Coverage Δ
asdf_astropy/converters/coordinates/angle.py	100.00% <100.00%> (ø)
...f_astropy/converters/coordinates/earth_location.py	100.00% <100.00%> (ø)
asdf_astropy/converters/coordinates/frame.py	100.00% <100.00%> (ø)
...f_astropy/converters/coordinates/representation.py	100.00% <100.00%> (ø)
asdf_astropy/converters/coordinates/sky_coord.py	100.00% <100.00%> (ø)
...f_astropy/converters/coordinates/spectral_coord.py	100.00% <100.00%> (ø)
asdf_astropy/converters/fits/fits.py	100.00% <100.00%> (ø)
asdf_astropy/converters/table/table.py	98.24% <100.00%> (+0.46%)	⬆️
asdf_astropy/converters/time/time.py	100.00% <100.00%> (ø)
asdf_astropy/converters/time/time_delta.py	100.00% <100.00%> (ø)
... and 14 more

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[braingram]

@WilliamJamieson Would you help me reproduce the segfault? Testing locally with several asdf version and extensions within asdf, custom extensions and ones in asdf-astropy I've so far failed to produce a segfault by changing the types and tags to tuples.

[WilliamJamieson]

@WilliamJamieson Would you help me reproduce the segfault? Testing locally with several asdf version and extensions within asdf, custom extensions and ones in asdf-astropy I've so far failed to produce a segfault by changing the types and tags to tuples.

You are correct, it was an independent bug I introduced.

[braingram]

Thanks for the follow up!