Add doc/requirements

Removing specific doc requirements from test-requirements.txt and move
them to a dedicated place.

Indeed, we need to specify doc requirements in doc/requirements.txt
to avoid problems with the pip resolver [1] for the release team [2][3].

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: Ibf838f01d935763ff1e0d19f10340c8ee360784c

doc/requirements.txt

@@ -0,0 +1,2 @@
+sphinx>=2.0.0,!=2.1.0 # BSD
+openstackdocstheme>=2.2.1 # Apache-2.0

test-requirements.txt

@@ -6,5 +6,3 @@ bashate>=0.5.1 # Apache-2.0
 hacking>=3.0.1,<3.1.0 # Apache-2.0
 
 docutils>=0.11 # OSI-Approved Open Source, Public Domain
-sphinx>=2.0.0,!=2.1.0 # BSD
-openstackdocstheme>=2.2.1 # Apache-2.0

tox.ini

@@ -18,6 +18,8 @@ whitelist_externals = bash
 commands = {toxinidir}/tools/run_bashate.sh
 
 [testenv:docs]
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -d doc/build/doctrees -b html doc/source/ doc/build/html
 
 [testenv:pep8]