scripts/stubtest: type-check it in tox -e type

Author: bluetech

tox.ini

@@ -49,7 +49,9 @@ commands = flake8 {posargs}
 [testenv:type]
 description = type check ourselves
 basepython = python3.7
-commands = python -m mypy --config-file mypy_self_check.ini -p mypy
+commands =
+    python -m mypy --config-file mypy_self_check.ini -p mypy
+    python -m mypy --config-file mypy_self_check.ini scripts/stubtest.py
 
 [testenv:docs]
 description = invoke sphinx-build to build the HTML docs