Apply upper constraints in doc builds

... according to the common practise.

Change-Id: I0759c775cb1364d005a3d29523a3fe1f60663d57
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

tox.ini

@@ -45,12 +45,18 @@ passenv =
 commands = {posargs}
 
 [testenv:docs]
-deps = -r{toxinidir}/doc/requirements.txt
-commands = sphinx-build -W -b html doc/source doc/build/html
+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:releasenotes]
-deps = -r{toxinidir}/doc/requirements.txt
-commands = sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
+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]
 builtins = _