bug: add missing Sun in VOUnits simple units

[ManonMarchand]

Hi astropy!

Description

The unit Sun is missing from the VOUnits simple_units list. This is documented in table 5 of the standard (version 1.1, from 2023/12/15) :

(and I checked with one on the authors of the standard)

This PR just adds it. This should solve astropy/astroquery#2480

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[pllim]

Thanks! Can you please add a test?

[ManonMarchand]

Sure! But I cannot find any existing tests on the simple units. What about something that asserts that they can be created without warnings but that adding a prefix raises a warning?

[pllim]

Something is better than nothing. I would recommend a test that could prevent your astroquery bug from creeping back in. Thanks!

[mhvk]

Thanks for the addition! Would indeed be good to have a test, could be as simple as adding the following to tests/test_format.py::test_vounit_details after the test for "pix":

    # Regression test for astropy/astroquery#2480 
    assert u.Unit("Sun", format="vounit") is u.Sun

EDIT: I actually think this is implicitly tested in the overall tests of VOUnits, but I agree with @pllim that we might as well be explicit for anything that caused a problem!

[ManonMarchand]

Thanks for the suggestion, done :)

[mhvk]

Nice! thanks, @ManonMarchand!