Prevent unexpected warnings in the code base

### Body Spun out of https://github.com/apache/airflow/pull/19725#issuecomment-977062213 It may be worthwhile to strengthen up the test suite to also catch warnings so we can fix them before the reach the user. Currently there are a ton of warnings being emitted from the test suite, and it’s not exactly optimal (and makes the Pytest result yellow instead of green). [Pytest has a flag for “strict mode”.](https://docs.pytest.org/en/6.2.x/warnings.html) I’m thinking maybe we can add a CI job to enable that, and slowly fix those warnings (either by actually rewriting relevant code, or just add suppression when they are expected). Once they are all delt with, we can enable them globally on all jobs (and remove the redundant job). ### Committer - [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.