Update deps for tox venv target

This makes the venv target use all of the requirements files
including docs/requirements so you can create a release note
using:

  $ tox -e venv -- reno new <slug>

Change-Id: Ic6e53d3aa10d48d56af2886e9dee59c52a9a2994

tox.ini

@@ -47,6 +47,11 @@ commands =
 local-check-factory = openstack._hacking.factory
 
 [testenv:venv]
+deps =
+    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+    -r{toxinidir}/test-requirements.txt
+    -r{toxinidir}/requirements.txt
+    -r{toxinidir}/doc/requirements.txt
 commands = {posargs}
 
 [testenv:debug]