It seems both astropy and the testing framework have started to draw in an inordinate number of dependencies. I'm quite happy with our own ones, as these are under our own control, but feel any beyond need an extremely strong rationale to be included, and if there is no such rationale, they can still be used but should be among the "optional tests" category. For me, yes, cov should be included, but I see no strong reason for either mock or hypothesis - indeed, the first test file that uses hypothesis has now caused multiple issues, partially because hypothesis is evolving fast. I do feel that with each new package, we're just spreading our development brains more thinly.

I'll open a PR if there is agreement that we should have core and optional testing dependencies.