explore switch to github actions

[emillipede]

github actions may offer a free way to test build changes (as part of PRs) and may be a viable alternative to circle CI (https://help.github.com/en/actions/automating-your-workflow-with-github-actions/events-that-trigger-workflows). github actions may also offer an alternative to locally running the build (in some circumstances)

two proposed workflows:

1. test
   -triggered on PR to master (mimics current circle CI, but should push to a test website)

2. production
   -triggered on push to master (mimics current circle CI, pushes to live site)
   -also triggered on set schedule

from @greysonp

Github Actions looks pretty neat, and has the crucial feature needed to handle nightly deploys: cron-style scheduling:
https://help.github.com/en/actions/automating-your-workflow-with-github-actions/events-that-trigger-workflows#scheduled-events-schedule

• look up pathlib in python 3?

• another consideration: at the moment, no tree data are updated on a regular basis (SM tree's inventory dataset was last updated March 2020). we might reconsider how often the API queries run.

see also #297 for github actions <> publictreemap.org