Fix GitHub Actions builds

[fedarko]

See #563.

• the main build is broken due to -- it looks like -- Q2 2020.6 not being installable any more, due to seaborn version stuff?
• the standalone python 3.7 build is broken due to using a now-old version of skbio (?)

I think the general approach will be updating the versions of the Python packages used (and making sure that this doesn't break empress). Per https://devguide.python.org/versions/, Python 3.7 and below are not even supported any more, so if getting those versions running is a pain we can probably just remove support for them (and the associated old Q2 versions that fix old Python versions; the most recently released? Q2 environment is already on Python 3.9).