Add doc/requirements.txt to venv tox target
Add doc/requirements.txt to venv tox target

This is needed to create a release note using the venv target:

  tox -e venv -- reno new <slug>

Change-Id: I1a0800a90781ede281b22db2892fb91e700d6e7b

stackforge/watcher,stackforge/watcher,openstack/watcher,openstack/watcher