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].  Removed 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 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: Ie0c8db6331630c5d3a379a9c8c2d87eb0a8fd207

doc/requirements.txt

@@ -0,0 +1,3 @@
+openstackdocstheme>=2.2.1 # Apache-2.0
+sphinx>=2.0.0,!=2.1.0 # BSD
+Pillow>=2.4.0 # PIL License

test-requirements.txt

@@ -1,11 +1,8 @@
 # needed for doc build
 docutils>=0.11,!=0.13.1  # OSI-Approved Open Source, Public Domain
-openstackdocstheme>=2.2.1 # Apache-2.0
-sphinx>=2.0.0,!=2.1.0 # BSD
 PyYAML>=3.1.0
 yamllint>=0.5.2
 requests-cache
-Pillow>=2.4.0 # PIL License
 hacking>=3.0.1,<3.1.0 # Apache-2.0
 whereto>=0.3.0  # Apache-2.0
 

tox.ini

@@ -54,6 +54,8 @@ commands =
 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 doc/source doc/build/html
 
 [testenv:validate]