Use bellybutton to encode various Airflow best practices

[sarayourfriend]

Problem

There are some subtle rules/conventions that our repository follows that could be automatically checked using https://github.com/hchasestevens/bellybutton.

Description

Add https://github.com/hchasestevens/bellybutton and create two new rules:

1. Prevent top level Variable.get (see Move API key Variable access to provider ingester init function #1457)
2. Disallow pendulum and to instruct the use of datetime instead (see Standardize on datetime over pendulum openverse-catalog#678)
3. Set sensor modes to rescheduled (see this review comment explaining why: Fix various issues that existed in the rotate_db_snapshots DAG #2158 (comment))

[AetherUnbound]

This is fantastic! 🤩 What a find! I love it, it'll be perfect for the conventions you point out (and probably others yet-to-be-identified).

[sarayourfriend]

I'm playing around with astpath, the library underneath Bellybutton, and I can't get it to select anything, not even basic nodes that I know exist like "Module" or "ClassDef".

I could be doing this wrong, of course. Bellybutton and astpath haven't been updated in a long time though, so I wouldn't be surprised if they're working off of old information.

[sarayourfriend]

Rather than bellybutton, we could use https://fixit.readthedocs.io/en/latest/ and only enable it for custom rules. It's custom rules even support fixes, which the sensor changes, for example, would be decent at providing in some cases.

Writing the rules along with tests in Python seems a lot easier and more straightforward to me than bellybutton and LibCST looks to be a bit of a better AST to work with as well.