On Ubuntu 18.04 (others not tested) installing the dev requirements
fails with the following error
babel 2.6.0 has requirement pytz>=0a, but you'll have pytz 2012d which
is incompatible.
Despite the comment in dev_requirements.txt pytz-2012d is not the most
recent version to support Python 2.6. In fact the latest release of pytz
supports Python 2.6.