newest xgcm version breaks CI for python<3.9

[jbusecke]

I just noticed that some of our tests for 3.8 break, and this seems to be related to xgcm 0.7.0.

I guess the easiest solution is to bump to >=3.9 for now. But ultimately the xgcm_utils module should probably be refactored into another package (possibly into xgcm). This would release the dependency on xgcm here I believe.

I am not sure how many folks are relying on that module at the moment, so I will open a separate issue and discuss that over there.

[jbusecke]

EDIT: Posted this in the wrong issue. The relevant issue is #140

Also cc'ing @TomNicholas here. Tom, there might be some bits of logic that might be helpful for xgcm.