HTTPretty: bump to 0.8.0

This new version brings Python 3 support, leaner requirements and better
packaging. Python-keystoneclient needs this to be Python 3 compatible.

Change-Id: I85298e7b99cfb1abec42058e63ee419e46900cb4
Merge "HTTPretty: bump to 0.8.0"
Merge commit 'refs/changes/81/68181/1' of git://gold.eng.hq.xensource.com/git/github/openstack/requirements into tmpbranch