Update CI virtual environments

[jrvanwhy]

As highlighted in #472, GitHub does not handle virtual environment turndowns well. Instead of causing quick CI failures, the action will hang for over a day, and the message about using an obsolete virtual environment is hidden pretty deep in the actions interface.

As far as I can tell, there's no way to get alerts for using deprecated virtual environments. Lacking that, we should probably switch to using -latest versions and accept the risk of sporadic breakage. We can always switch back if the sporadic breakage is worse than occasionally using an obsolete virtual environment.