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: I813c8d5c20b576911baea15fe9ae106e74500f3b

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
+doc8>=0.6.0 # Apache-2.0

test-requirements.txt

@@ -3,9 +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
-doc8>=0.6.0 # Apache-2.0

tox.ini

@@ -25,12 +25,15 @@ commands = {posargs}
 commands = python setup.py test --coverage --testr-args='{posargs}'
 
 [testenv:docs]
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf doc/build
   sphinx-build -W --keep-going -b html doc/source doc/build/html
   doc8 --ignore D000 -e .rst doc/source/ CONTRIBUTING.rst HACKING.rst README.rst
 
 [testenv:releasenotes]
+deps = {[testenv:docs]deps}
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html