Add doc/requirements

We need to specify doc requirements in doc/requirements.txt
to avoid problems with the pip resolver [1] for the release team [2][3].
Removing specific doc requirements from test-requirements.txt.

The problem here is that this repos haven't doc/requirements.txt file
and by default in this case zuul will use the test-requirements.txt file
to pull requirements [4].

This requirements file contains extra requirements like flake8 that
collided with those allowed in our job environment and so the new pip
resolver fails to install these requirements and the job exits in error.

This project meet the conditions leading to the bug.

[1] http://lists.openstack.org/pipermail/release-job-failures/2021-January/001500.html
[2] http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019611.html
[3] http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019612.html
[4] https://opendev.org/zuul/zuul-jobs/src/branch/master/roles/ensure-sphinx/tasks/main.yaml#L36

Change-Id: I4bbbd504e7be70e1a02b308c5e7b2a761e9d124f

doc/requirements.txt

@@ -0,0 +1,4 @@
+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

@@ -3,8 +3,3 @@
 # process, which may cause wedges in the gate later.
 
 hacking>=3.0.1,<3.1.0;python_version>='3.5' # Apache-2.0
-
-sphinx>=2.0.0,!=2.1.0 # BSD
-openstackdocstheme>=2.2.1 # Apache-2.0
-# releasenotes
-reno>=3.1.0 # Apache-2.0

tox.ini

@@ -35,9 +35,12 @@ 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