Add type checking

[kaapstorm]

Technical Summary

Adds a Github workflow for mypy to check the type hints that already exist in the codebase.

Only modules listed in mypy_modules.txt are checked, and type-checking ignores functions and methods in the module that don't have type hints. (In other words, type hints are treated the same as doctests: Not all functions or methods in a module need to have doctests, and modules that do have doctests need to be added to a test suite for them to be tested.)

Safety Assurance

Safety story

There is only one code change: Fixes a bug, exposed by type checking, when flower_url is not set.

Rollback instructions

• This PR can be reverted after deploy with no further considerations

Labels & Review

• Risk label is set correctly
• The set of people pinged as reviewers is appropriate for the level of risk of the change