Run Mypy static checks against our tests. This brings them inline with
the rest of the codebase, and we have an extra level of certainty that
the tests (and unit tests in particular) will not significantly diverge
from the code they are testing.
static methods that are used application-wide should belong to
the utilities python file.
inspired by @gmarmstrong's PR #166 on refactoring global_common
methods to be static and have a dzutil.py