Update to latest miniconda and test in OSX

[sjanssen2]

Is there a reason why we are using relatively old software versions for e.g. Ubuntu, Miniconda2 instead of 3, ... ?

empress/.travis.yml

Line 4 in 7cd33d2

dist: trusty

Shouldn't we add testing on OSX?

[fedarko]

Now that we've transitioned to using GitHub Actions (main CI, standalone CI), it looks like we are using the latest Ubuntu (and I think conda) versions.

So I think all that is left for this issue is adding testing on macOS systems -- it looks like this is very doable with GitHub Actions (there is an example of this about midway down that page).