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: I4ff8cee25887d38e18fd260560b92ad4bf904838

doc/requirements.txt

@@ -0,0 +1,3 @@
+sphinx>=2.0.0,!=2.1.0 # BSD
+openstackdocstheme>=2.2.1 # Apache-2.0
+reno>=3.1.0 # Apache-2.0

test-requirements.txt

@@ -6,8 +6,3 @@ oslotest>=3.8.0 # Apache-2.0
 testscenarios>=0.4 # Apache-2.0/BSD
 requests-mock>=1.2.0 # Apache-2.0
 keystoneauth1>=3.4.0 # Apache-2.0
-
-# docs, releasenotes
-sphinx>=2.0.0,!=2.1.0 # BSD
-reno>=3.1.0 # Apache-2.0
-openstackdocstheme>=2.2.1 # Apache-2.0

tox.ini

@@ -33,10 +33,15 @@ commands =
   coverage report
 
 [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:releasenotes]
+envdir = {toxworkdir}/docs
+deps = {[testenv:docs]deps}
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html