Fix the python-dateutil dependency for Python 2.7.
Review Request #11391 — Created Jan. 20, 2021 and discarded
The recent change to limit dependency versions for Python 2.7 was a
little bit too eager, and ended up installing a version of dateutil that
python-dateutilonly dropped support for
Python 2 as of the most recent version 2.8, so we can keep using up to
Installed dependencies on Python 2.7. Verified that this fixed an issue
that a student was encountering.