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: I408b1fa83322fa39a9f9d6dfaed5174ed5a74ae7

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
@@ -44,7 +46,7 @@ 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