This makes mypy about 15% faster running on the Zulip codebase (from
7s=>6s on my laptop), which seems worth it for losing a couple files.
This option requires a new dependency, which we add to the
mypy-specific requirements.txt file.
Also update the mypy command line to not use deprecated argument names.
This introduces a few errors, so we exclude the relevant files to keep
the mypy output clean.