Run doctests only when using pinned requirements for most recent Python version

[namurphy]

Description of improvement

Instead of running doctests every time we run tests, we should consider switching to running doctests only on the pinned requirements in requirements.txt that get updated via an automated PR every week.

Motivation

Sometimes upstream packages like NumPy and Astropy change how they print out things like ndarray and Quantity objects. If we ran doctests only on a single updated set of package versions, we wouldn't have to do any version-dependent skipping of doctests.

Since we currently run code coverage on an older version of the tests, a consequence of this change would be that lines of code covered only by doctests would no longer show up as being covered by tests. This would help us avoid situations where certain lines of code are only run during doctests and not in our standard test suite, or a function is only tested in doctests.

Implementation strategy

The change would be in tox.ini and/or .github/workflows/tests.yml. Might be easiest to try getting ChatGPT to do this one for us. It would probably involve changing the flags provided to pytest.

Additional context

See also #135, #229, #653, #2148, astropy/astropy#14553, astropy/astropy#15095, astropy/astropy#15312

[namurphy]

Closed by #2523.