Add pytz to requirements

The default behavior as of pip 1.4 is to treat any version containing substrings like a, alpha, b, beta, rc, etc. as a prerelease, which will not be installed without adding the --pre flag to the pip command line.

So, localshop depends on django-celery, which depends on pytz. As pytz is versioned with year and a letter (e.g. 2013b), the new version of pip will not install it by default. This breaks my automation (https://github.com/needle-cookbooks/chef-localshop) and is generally frustrating.

Adding a version constraint for pytz to requirements.txt fixes this right up.