deprecate asdf.tests.helpers

[braingram]

The current users of asdf.tests.helpers (outside this repo) are (with 1 example from each):

• astropy.io.misc.asdf
• specutils: https://github.com/astropy/specutils/blob/75a6cba8f7465878823d25e1ddf3ea47c0e73051/specutils/io/asdf/tags/tests/test_spectra.py#L11
• asdf-astropy: https://github.com/astropy/asdf-astropy/blob/706f4ec19d6509966f363c3ade6a4cfd46c4a15d/asdf_astropy/converters/unit/tests/test_unit.py#L5

The uses in the deprecated astropy.io.misc.asdf are ignored.

specutils uses assert_roundtrip_tree to test that object serialization then deserialization results in an equivalent object. asdf.testing.roundtrip_object can be used but requires either using the assert_equal class method defined on the spectra, implementation of an equality operator or a similar test to verify that the deserialized object is 'equal' (it currently relies on a feature of assert_roundtrip_tree which checks for the assert_equal class method of the custom type).

asdf-astropy uses yaml_to_asdf. An alternative, drop in replacement exists in asdf.testing.helpers.

[WilliamJamieson]

specutils uses assert_roundtrip_tree to test that object serialization then deserialization results in an equivalent object. asdf.testing.roundtrip_object can be used but requires either using the assert_equal class method defined on the spectra, implementation of an equality operator or a similar test to verify that the deserialized object is 'equal' (it currently relies on a feature of assert_roundtrip_tree which checks for the assert_equal class method of the custom type).

asdf-astropy uses yaml_to_asdf. An alternative, drop in replacement exists in asdf.testing.helpers.

If this is the case, please open PRs on specutils and asdf-astropy implementing these fixes. I would like to review those changes first before I consider the changes in this PR.

Also, have you checked the if the new warnings are raised in the rest of the downstream tests? The three packages you list here are simply the ones which turn warnings into errors. I would like to cover off these changes in the rest of the downstream.

[WilliamJamieson]

After an offline conversation with @braingram, it was explained that his analysis indicates that they only downstream uses of asdf.tests.helpers were in astropy, specutils, and asdf-astropy which already have solution paths planned.

Given this analysis, I retract my previous comments (aside from removing the single suppression from the downstream tox).

These changes look good to me otherwise.

[braingram]

Fixes #1363

[braingram]

If this is the case, please open PRs on specutils and asdf-astropy implementing these fixes. I would like to review those changes first before I consider the changes in this PR.

@WilliamJamieson I opened PRs:
astropy/asdf-astropy#168
astropy/specutils#1025

I converted them both to draft in case we'd like to rename testing.helpers. I'm inclined to leave it as is but don't feel strongly. If you'd like to change the name I don't have any objection.