CI: Run mypy with --check-untyped-defs (MR 2431)

All typing errors found with --check-untyped-defs have been fixed, so
let's make mypy stricter now to avoid regressing this.

.ci/mypy.sh

@@ -11,4 +11,4 @@ fi
 set -x
 
 pip install --break-system-packages --no-warn-script-location mypy
-python -m mypy --color-output pmbootstrap.py
+python -m mypy --color-output --check-untyped-defs pmbootstrap.py