[MLGO] Add pytype test (#79558)

This patch adds a pytype test to the mlgo-utils lit test suite to
prevent regressions in typing while we wait to setup precommit CI for
this specific project.

This is somewhat hacky, but is a temporary fix and will be pulled out
soonish. It also should only impact the ML buildbots as they are the
only ones that should have `pytype` installed.