Run with mypy --check-untyped-defs

pyproject.toml

@@ -63,6 +63,7 @@ plugins = ["mypy_django_plugin.main"]
 disallow_any_generics = true
 no_implicit_reexport = true
 warn_return_any = true
+check_untyped_defs = true
 
 [[tool.mypy.overrides]]
 # Don't follow the import chain into the module containing vendored job-server