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].
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 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: Ia9616b9c14d59ff382509086fcbf00f56a0701ea

doc/requirements.txt

@@ -0,0 +1,4 @@
+openstackdocstheme>=2.0.0  # Apache-2.0
+
+sphinx>=2.0.0,!=2.1.0 # BSD
+reno>=2.5.0  # Apache-2.0

test-requirements.txt

@@ -1,5 +1 @@
-openstackdocstheme>=2.0.0  # Apache-2.0
-
-sphinx>=2.0.0,!=2.1.0 # BSD
-reno>=2.5.0  # Apache-2.0
 jinja2

tox.ini

@@ -29,6 +29,8 @@ deps = bindep
 commands = bindep test
 
 [testenv:releasenotes]
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/doc/requirements.txt
 basepython = python3
 whitelist_externals = bash
 commands = bash -c ci-scripts/releasenotes_tox.sh