Create a tox env for requests v1.2.3

Requests 1.2.3 seems to be a real pain, it's right before the v2.0
cleanup and there have been a number of inconsistencies. Create a tox
environment for checking them, and we should really gate on this as
well.

Change-Id: I343b9b7124a6bfb8e1fb711a286b8a896f50fc7b
Merge "Create a tox env for requests v1.2.3"