www/py-django: Add missing runtime dependency

pytz is now a required dependency for Django 1.11 and is strongly
recommended in older releases.

Django 1.11:

https://github.com/django/django/commit/414ad25b090a63eaaf297b1164c8f7d814a710a2

Older releases: docs/topics/i18n/timezones.txt

"3. Should I install pytz?
Yes. Django has a policy of not requiring external dependencies, and for
this reason pytz_ is optional. However, it's much safer to install it."

Reviewed by: lwhsu
MFH: 2017Q3
Differential Revision: https://reviews.freebsd.org/D11835