Drop support for NumPy 1.20

[WilliamJamieson]

Description

Per APE 18, as of today (January, 31 2023) we should drop support for NumPy 1.20 (see https://numpy.org/neps/nep-0029-deprecation_policy.html#drop-schedule). This PR executes that drop of support

[github-actions]

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[mhvk]

With dropping the support, we should also remove work-arounds that are needed for numpy 1.20. I think there are not so many:

find . -name \*.py -exec grep NUMPY_LT_1_21 {} \; -print
    "NUMPY_LT_1_21_1",
NUMPY_LT_1_21_1 = not minversion(np, "1.21.1")
./astropy/utils/compat/numpycompat.py
                # perhaps axis or where was masked (in NUMPY_LT_1_21 this is
./astropy/utils/masked/core.py
from astropy.utils.compat import NUMPY_LT_1_21_1, NUMPY_LT_1_22
        sys.byteorder == "big" and NUMPY_LT_1_21_1, reason="Numpy GitHub Issue 19153"
./astropy/units/tests/test_quantity_array_methods.py
from astropy.utils.compat import NUMPY_LT_1_21_1
    @pytest.mark.skipif(NUMPY_LT_1_21_1, reason="https://stackoverflow.com/q/69571643")
./astropy/units/tests/test_structured.py

Since these are all "mine", I thought I might as well fix them... Will push a commit shortly, and approve assuming tests pass.

[pllim]

@mhvk , won't this comparison also hold for 1.21.0?

[mhvk]

Darn, I'm an idiot! I just thought we were dropping support for the oldest numpy version in numpycompat.py, but apparently we never had anything particularly bad about numpy 1.20... Sigh...

And thanks for catching that. Am happy to push an update with that commit removed...

[mhvk]

I removed my incorrect adjustments! Should now all be fine.

[mhvk]

Failures in -dev are all unrelated (asdf), so let's get this in. Thanks, @WilliamJamieson, and apologies again for adding edits for the wrong numpy version!