[pytype] Disable mypy type checking on third_party/mako

Change-Id: Ia1e0f715635d08bcf8153129906a49ebe4bb9a03
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mako/+/1117866
Reviewed-by: Ina Huh <ihuh@google.com>