Test against minimum supported version of sunpy

[dstansby]

Describe the feature

In the Python 3.7 builds we should test against the minimum supported version of sunpy. This is currently claimed to be sunpy 2.1, but we can bump this a bit if there are compatability issues. I do not want to bump the minimum version for the sake of bumping it though - we should check if we need to first.

Proposed solution

No response

[nabobalis]

We only run devdeps: a oldestdeps build needs to be added.