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: I9b10cd04a98d4ec2899962c53bda16ec860b2c4d

doc/requirements.txt

@@ -0,0 +1,5 @@
+sphinx>=2.0.0,!=2.1.0 # BSD
+openstackdocstheme>=2.2.1 # Apache-2.0
+
+# This is required for the releasenotes build jobs
+reno>=3.1.0 # Apache-2.0

test-requirements.txt

@@ -5,9 +5,4 @@
 hacking>=3.0.1,<3.1.0 # Apache-2.0
 oslotest
 
-sphinx>=2.0.0,!=2.1.0 # BSD
-openstackdocstheme>=2.2.1 # Apache-2.0
 testrepository>=0.0.18
-
-# This is required for the releasenotes build jobs
-reno>=3.1.0 # Apache-2.0

tox.ini

@@ -26,4 +26,6 @@ exclude = .venv,.tox,dist,*.egg,build,releasenotes
 show-source = true
 
 [testenv:releasenotes]
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/doc/requirements.txt
 commands = bash -c tools/releasenotes_tox.sh