Add github action that runs unittest and linter

[manulera]

Hello, I think it would be good to have a github action that runs the tests on PR or Push, and runs a linter. Happy to contribute this during Harmony. @jakebeal

Do you have a problem with using black as a linter?

[jakebeal]

We do have an action for push & PR, it had just switched itself to idle since nobody's done a push or PR for a couple of months. I've turned it on again.

For running a linter, we've found black to be too restrictive. I'd prefer to match pysbol3, which usees pycodestyle and pylint with the following configuration: https://github.com/SynBioDex/pySBOL3/blob/main/setup.cfg
Note that this might find a massive number of issues, since it hasn't been run recently, so we'll likely want to see how bad it is before deciding how to best proceed, in terms of time prioritization.