protobuf: build with up-to-date dateutil

Fixes #32571. @mistydemeo, can you take a look / try this?
We're already doing it implicitly; setup.py will just go grab missing packages and install them into prefix anyway. Which is maybe fine.

If you pip install python-dateutil (which installs 2.2) and then pip install google-apputils, pip will just quietly remove your dateutil and replace it with 1.5. setup.py isn't as aggressive about resolving requirements. [PEP 453](http://legacy.python.org/dev/peps/pep-0453/#recommendations-for-downstream-distributors) suggests we should be running pip, anyway, so maybe that's a solution, though in this case it'll just downgrade users' dateutils for probably no real reason.