TST: Drop deprecated astropy.tests.helper.raises

[rrjbca]

Description

The latest astropy development version has deprecated astropy.tests.helper.raises. This raises a warning in our compatibility workflow e.g. HERE. Following the astropy recommendation, this PR drops astropy.tests.helper.raises and uses pytest.raises instead.

Checklist

• [x] Follow the Contributor Guidelines
• [ ] Write unit tests
• [ ] Write documentation strings
• [ ] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request