Deprecate catch_warnings and raises in favor of pytest machinery?

[pllim]

For example, some catch_warnings can be replaced with pytest.warns (see https://docs.pytest.org/en/latest/warnings.html#warns). As a result, do we still need warning related functionality in astropy/tests/helper.py?

[mhvk]

I've wondered too whether pytest has caught up with our custom machinery; it would certainly be nice to be as compliant as possible in our tests.

[pllim]

Started an attempt at #10504 .

UPDATE: With that PR merged, we no longer use catch_warnings or raises from helper.py but it doesn't stop them from creeping back in until we start to deprecate them. Deprecation is a battle for another day.