MNT: Refactor the way astropy.utils.data is monkeypatched in tests

Over the years, every time `astropy/utils/data.py`, particularly its `get_pkg_data_fileobj` function, is refactored, it tends to break the tests here in `astroquery`. It would be good to consider refactoring the way you mock or monkeypatch this code to avoid constant patching of the monkeypatch.

Examples: #2059 #1711

Start by reviewing tests that mock or monkeypatch astropy.utils.data.py, particularly get_pkg_data_fileobj, and compare the breakages described in issues #2059 and #1711. Determine a less fragile test boundary; done means upstream refactors of that function no longer require repeatedly updating astroquery's monkeypatching tests.

Written by the indexing model from the issue text.