mrhlpr, mrtest: Add lots of type hints and fix type errors + CI: Run mypy with --disallow-untyped-calls

frontend.main() only ever returns None, so there's no point in calling sys.exit with its return value as parameter.

.ci/mypy.sh

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