CI: Add --check-untyped-defs to mypy check

This should reduce the risk of changes being missed in refactors like
the one discovered by Zach DeCook which was fixed in
7fce8005.

.ci/mypy.sh

@@ -10,4 +10,4 @@ fi
 
 set -x
 
-mypy mrhlpr.py mrtest.py
+mypy mrhlpr.py mrtest.py --check-untyped-defs