Specify minimum version of pytz

pytz has unconventional version numbers which cause problems with
newer versions of pip.  Specifying a minimum version bypasses these
issues.

See
http://stackoverflow.com/questions/18230956/could-not-find-a-version-that-satisfies-the-requirement-pytz

This change was made in the main development branch as
5491610c235d84847da4836bcdd98cb05823c7c3