Add pytest-mpl back as a dependency here

[drdavella]

This would allow pytest-mpl to be automatically distributed with astropy's other test dependencies.

[bsipocz]

Oh, and pytest-arraydiff, too

[astrofrog]

👍

[pllim]

We took pytest-mpl back out because it pulls in matplotlib as a required dependency, which is undesirable. Once @astrofrog fixes that to make matplotlib optional again, it should be added back in (again).

xref astropy/astropy#9726 (comment)

[astrofrog]

Also xref matplotlib/pytest-mpl#87

[pllim]

Is it time to revisit this or should we close it?

[astrofrog]

We can revisit, I think matplotlib should no longer be a dependency

[pllim]

Are you sure? matplotlib is still required:

https://github.com/matplotlib/pytest-mpl/blob/2611bd020b523ab4fff1c9fa7db936b68dad7113/setup.cfg#L30