pytz not installed in virtualenvs

[gdub]

$ PYTHONPATH=/django /home/vagrant/.virtualenvs/py2.7/bin/python
Python 2.7.3 (default, Aug  1 2012, 05:14:39) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pytz
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named pytz

This is causing some tests to be skipped, e.g.

test_naturalday_uses_localtime (django.contrib.humanize.tests.HumanizeTests) ... skipped u'this test requires pytz'