Type-check Pandas code and require newer pyarrow.

[thetorpedodog]

• Pandas now has canonical type stubs, which mypy can find and install.
• We no longer support py3.7, so we don't need the pyarrow hotfix.
• Type annotation fix related to installing Pandas stubs.

[thetorpedodog]

To do this, I (after some trial and error):

1. Removed all the entries from the [[tool.mypy.overrides]] module = [...] setting, so that mypy would no longer ignore any modules
2. Ran update-requirements-txt to rebuild the requirements files.
3. Looked at the mypy output from update-requirements-txt and re-enabled ignoring modules that had no type stubs (and also fixed any issues that arose).