Add doc/requirements

This project isn't released but uniformization can't hurt.

The origin of these changes was that we need to specify doc requirements
in doc/requirements.txt to avoid problems with the pip resolver [1]
for the release team [2][3].

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 could meet similar 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: I0101b340fef5ae465eb64e7d7b21973d36fd2a6a

doc/requirements.txt

@@ -0,0 +1,2 @@
+sphinx>=2.0.0,!=2.1.0 # BSD
+openstackdocstheme>=2.2.1 # Apache-2.0

test-requirements.txt

@@ -3,6 +3,4 @@
 # process, which may cause wedges in the gate later.
 # needed for doc build
 docutils>=0.11,!=0.13.1  # OSI-Approved Open Source, Public Domain
-sphinx>=2.0.0,!=2.1.0 # BSD
-openstackdocstheme>=2.2.1 # Apache-2.0
 yamllint

tox.ini

@@ -13,6 +13,8 @@ basepython = python3
 commands = {posargs}
 
 [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 -d doc/build/doctrees doc/source doc/build/html
 
 [testenv:linters]