Remove explicit dateutil pin to silence DeprecationWarning

[shelman]

When running the tests locally, the output of make test contained the warning

=============================== warnings summary ===============================
/usr/local/lib/python3.7/site-packages/dateutil/parser.py:587
  /usr/local/lib/python3.7/site-packages/dateutil/parser.py:587: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
    if (isinstance(tzinfos, collections.Callable) or

-- Docs: https://docs.pytest.org/en/latest/warnings.html
===================== 5 passed, 1 warnings in 0.45 seconds =====================

I noticed that dateutil is not used anywhere in the first-party code - by removing the explicit pin I was able to silence the warning, which makes the pytest output less noisy.