Fix docs reqs for new pip

Per [1], the docs requirements were failing (or at least could)
due to test-requirements being used instead of doc/requirements.
The former may include dep versions not compatible with the
upper-constraints and hence cause issues for the new pip resolver.
This patch fixes that.

[1] http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019611.html

Change-Id: I2441e77bef1a1d2a9acdd6e42d3b2610e9ad8283

doc/requirements.txt

@@ -0,0 +1,5 @@
+sphinx>=2.0.0,!=2.1.0 # BSD
+openstackdocstheme>=2.2.1 # Apache-2.0
+
+# releasenotes
+reno>=3.1.0 # Apache-2.0

test-requirements.txt

@@ -7,13 +7,8 @@ hacking>=3.0.1,<3.1.0 # Apache-2.0
 coverage!=4.4,>=4.0 # Apache-2.0
 ddt>=1.0.1 # MIT
 python-subunit>=1.0.0 # Apache-2.0/BSD
-openstackdocstheme>=2.2.1 # Apache-2.0
 oslotest>=3.2.0 # Apache-2.0
 requests-mock>=1.2.0 # Apache-2.0
-sphinx>=2.0.0,!=2.1.0 # BSD
 stestr>=1.0.0 # Apache-2.0
 testscenarios>=0.4 # Apache-2.0/BSD
 testtools>=2.2.0 # MIT
-
-# releasenotes
-reno>=3.1.0 # Apache-2.0

tox.ini

@@ -31,10 +31,13 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
+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 = {[testenv:docs]deps}
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html