Add types-python-dateutil to dev requirements

The previous commit began using a submodule of the dateutil builtin
python library. We weren't previously using this library, and thus didn't
need the type stubs for it. But now that we do use it, we need to have
the type stubs during development.

dev-requirements.txt

@@ -34,6 +34,7 @@ types-PyYAML
 types-Jinja2
 types-mock
 types-protobuf>=5.0,<6.0
+types-python-dateutil
 types-pytz
 types-requests
 types-setuptools