python3: Work around httpretty issue

Currently running tox.ini with py33 installation
fails because of a failure to read a unicode file
while installing httpretty.

Closes-Bug: #1241560

Change-Id: I6012c4fcca0d0354f0c1bdf4797f6e7409f7bf1c
Signed-off-by: Chuck Short <chuck.short@canonical.com>
Merge "python3: Work around httpretty issue"