This module doesn't exist, and never did; the name appears to be a
mistaken variant of the module that really does contain ZulipTestCase.
So, fix the import to use the real name.
This would never have worked at runtime, which is why it's in an
`if False:`. It's also an example of the kind of error that can be
hidden by `ignore_missing_imports`; we'd have caught the issue
immediately if we hadn't had a blanket application of that flag
in place.
This puts all of this config in one place, and also needs a lot fewer
lines to describe it; which, combined, makes it a lot clearer what our
normal config actually is. (I'd been looking at this script for a few
minutes without realizing that we have `--disallow-untyped-defs` *on*
by default, not off.)
Experimenting with different values is still easy; just comment the
line in the config.
This option "shows errors for missing return statements on some
execution paths." according to the mypy docs.
It doesn't currently bring up anything, after cleaning the cache.