Can you please change this to 'pytz >= 2013b'?

Pinning to the exact version is not future-friendly: new pytz's will come out, and people will want to use them, and we don't want to keep making forced zope.testbrowser releases for every new pytz release.

I've verified on the command line that `pip install 'pytz>==2010o'` (where 2010o is just some old version I picked at random) correctly installs the latest version, 2013b.

Incidentally, upstream is trying to solve this pytz problem once and for all in https://bugs.launchpad.net/pytz/+bug/1204837 and https://github.com/pypa/pip/issues/974.