Fix polar scatter serialization issue

[Carifio24]

This PR fixes an issue with saving glue sessions that have polar plots in degree mode. This is accomplished by creating the RadianTransform class in roi_pretransforms.py, which is glue-serializable. This replaces the previous transform that was applying the conversion to degrees (an instance of functools.partial), which glue does not know how to serialize.

[codecov]

Codecov Report

Merging #2259 (a89a3dd) into main (bdffcca) will decrease coverage by 0.05%.
The diff coverage is 44.44%.

❗ Current head a89a3dd differs from pull request most recent head 6eba0b5. Consider uploading reports for the commit 6eba0b5 to get more accurate results

@@            Coverage Diff             @@
##             main    #2259      +/-   ##
==========================================
- Coverage   88.05%   88.00%   -0.06%     
==========================================
  Files         247      247              
  Lines       23156    23165       +9     
==========================================
- Hits        20391    20386       -5     
- Misses       2765     2779      +14     

Impacted Files	Coverage Δ
glue/core/roi_pretransforms.py	76.92% <40.00%> (-23.08%)	⬇️
glue/viewers/scatter/viewer.py	98.91% <50.00%> (+1.97%)	⬆️
glue/core/state.py	92.12% <100.00%> (ø)
glue/_mpl_backend.py	80.00% <0.00%> (-8.00%)	⬇️
glue/dialogs/data_wizard/qt/data_wizard_dialog.py	91.46% <0.00%> (-2.44%)	⬇️
glue/utils/qt/app.py	94.00% <0.00%> (-2.00%)	⬇️
glue/conftest.py	53.03% <0.00%> (-1.52%)	⬇️
glue/utils/qt/helpers.py	53.93% <0.00%> (-1.13%)	⬇️

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[astrofrog]

Looks good, thanks! Could you add a changelog entry?

[Carifio24]

Added the changelog entry