Add an import-only test matrix entry

#7932 / #7938 are about a subtle bug that came from accidentally implicitly creating a pytest dependency. Currently the test suite misses that case because the tests require pytest, so you can’t get around installing pytest.

But one could imagine a fairly simple script that installs numpy, cython, and jinja, builds, and imports astropy and all the sub-packages without running any tests. That wouldn’t add too much time and would catch any “implcit dependency” problems a PR might introduce…

I think I may have seen this idea years back and its conceivable there’s a duplicate issue about this before… But I couldn’t find it.

[drdavella]

I agree with this PR and think it’s the right thing to do. We should probably strive to be purists about our dependencies. 👼

On the other hand, part of me wonders what’s so bad about having pytest as a runtime dependency if we just declare it as such in setup.py? 😈

[pllim]

Maybe related to #10246