Dep's should be restricted by tox-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 tox-constraints.txt

Change-Id: Ia380129639046c2289c3e9a2a91971c988fd61ad

tox.ini

@@ -33,7 +33,9 @@ setenv =
     WORKING_DIR={toxinidir}
 
 [testenv:docs]
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
 commands =
     bash -c "rm -rf doc/build"
     doc8 doc
@@ -52,7 +54,7 @@ commands =
 extensions = .rst
 
 [testenv:releasenotes]
-deps = -r{toxinidir}/doc/requirements.txt
+deps = {[testenv:docs]deps}
 commands =
     sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html