Dep's should be restricted by upper-constraints

Tox trying to install latest versions for building docs which may
not be supported by stable and lower branches, so should be
restricted by respective version's upper-constraints.txt

Change-Id: I599d4ea430cc1e65bb50d0481617b005febf6de2
(cherry picked from commit ee641dffc9)

tox.ini

@@ -90,7 +90,9 @@ commands = {posargs}
 
 [testenv:docs]
 basepython = python3
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/victoria}
+  -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:api-ref]
@@ -141,7 +143,9 @@ commands = bindep test
 
 [testenv:releasenotes]
 basepython = python3
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/victoria}
+  -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -W -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:lower-constraints]