Use upper-constraints in docs job

Since we use usedevelop=True, as a result neutron-fwaas
installation happens at 'develop-inst' phase and it installs
requirements from requirements.txt without considering
tox constraints, This can lead to issue where broken
requirements are installed in docs job and as a result
it will fail.

In order to get constraints be considered for this case, add
requirements.txt to deps for docs env(same as [1]).

[1 https://review.opendev.org/q/Ic8f6472fe708d12f8fc129729465367083e1095a

Change-Id: Iea9556f6f025ad2f3dfa38fd401aa64414bd561f

tox.ini

@@ -122,7 +122,13 @@ commands = {posargs}
 deps = -r{toxinidir}/doc/requirements.txt
 
 [testenv:docs]
-deps = -r{toxinidir}/doc/requirements.txt
+# Do not remove requirements.txt from deps list as without it
+# upper constraints will not be used for deps listed in requirements.txt
+# and may cause issues
+deps =
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
+  -r{toxinidir}/requirements.txt
 allowlist_externals =
   rm
 commands =