Turn on the `--check-untyped-defs` mypy flag on all python modules

One caveat of mypy is that, by default, it only type checks functions that have type annotations.

Most of our tests do not have type annotations, and therefore their contents are not typechecked. Adding -> None to all tests feels like avoidable busywork.

We should consider turning on --check-untyped-defs on modules.

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Migrated from T2307 (view on Phabricator)