Bach/Modelhub tests: run on one version, and test 3.10

[thijs-obj]

Situation:

• BigQuery tests can be a bit unstable. Especially tests that drop and create tables in one statement can fail.
• We currently test python 3.7, 3.8, and 3.9, but not 3.10

Idea (not set in stone, open to other ideas):

• On every push: test on python 3.7
• On push to master: test on python 3.7, 3.8, 3.9, and 3.10