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: Iac559a62dcadadc409c643968b72e28e11d6d00c

doc/requirements.txt

@@ -0,0 +1,4 @@
+sphinx>=2.0.0,!=2.1.0 # BSD
+sphinxcontrib.autoprogram
+openstackdocstheme>=2.2.1 # Apache-2.0
+reno>=3.1.0 # Apache-2.0

test-requirements.txt

@@ -5,12 +5,6 @@
 hacking>=3.0.1,<3.1.0 # Apache-2.0
 
 coverage>=4.0,!=4.4 # Apache-2.0
-sphinx>=2.0.0,!=2.1.0 # BSD
 oslotest>=1.10.0 # Apache-2.0
 testrepository>=0.0.18  # Apache-2.0/BSD
 testtools>=1.4.0 # MIT
-# releasenotes
-reno>=3.1.0 # Apache-2.0
-
-sphinxcontrib.autoprogram
-openstackdocstheme>=2.2.1 # Apache-2.0

tox.ini

@@ -35,10 +35,13 @@ commands = bindep test
 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 =
    sphinx-build --keep-going -b html -W doc/source doc/build/html
 
 [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