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: Iecd29788d1c94fd727072aa5dd7fe5d828dbf174

tox.ini

@@ -51,7 +51,9 @@ commands =
     coverage report
 
 [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 = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
@@ -64,7 +66,9 @@ commands =
   make -C doc/build/pdf
 
 [testenv:releasenotes]
-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 = sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [flake8]