Tests went from passing to failing with no code change

[gully]

The most recent commit failed, despite the only change being a tutorial. How could that happen? I am investigating! My first guess would be a dependency change, but we have a matrix of astropy/specutils/python dependencies that all went from passing to failing. It's not numpy because that was the same between the two passing and failing commits. Strange!

[gully]

Something appears to have broken with astropy units between commit 3221bcf and the current HEAD. A workaround for @joelburke02 was to checkout this older commit.

[gully]

Ah-ha! I have found the problem. The upstream dependency gwcs caused a breaking change between version 0.16 and 0.17. The problem was identified in this astropy issue:
astropy/specutils#898

As a workaround I have pinned the gwcs dependency, and in the process also pinned the ndcube dependency. We'll want to revisit these choices as the upstreams get changed. Closing for now!