CI: run pytest, and run the build workflow on every pull request #26

The build workflow (.github/workflows/python-build.yml) runs make format-check and make typecheck only. It never runs make test, so the offline tests added in #21 are not checked by CI.

The workflow also triggers only on pull requests into main. Stacked PRs such as #21 get no checks unless the workflow is started by hand.

Follow-up: add a test step that runs the offline tests (the live tests need credentials, see the companion issue on skipping them), and trigger the workflow on all pull requests.