HTTPretty: update to 0.7.1

The 0.7.1 release has a fix for the parse_querystring function, which raised a
TypeError on Python 3.3. See:

https://github.com/gabrielfalcao/HTTPretty/commit/5c6e776

This helps fix some tests on Python3.3 in python-keystoneclient.

Change-Id: If14a0afe9d42602e978377d6f6ee830affe0cc41