Add doc/requirements

Move specific doc requirements from test-requirements.txt to
doc/reqs.txt.

This project isn't released but uniformization can't hurt.

The origin of these changes was that 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 could meet similar 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: Id7375754cc62bb730af6513c0af17cedcbbedacf

doc/requirements.txt

@@ -0,0 +1,2 @@
+openstackdocstheme>=1.31.2  # Apache-2.0
+sphinx>=1.5.0  # BSD

test-requirements.txt

@@ -2,5 +2,3 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 # needed for doc build
-openstackdocstheme>=1.31.2  # Apache-2.0
-sphinx>=1.5.0  # BSD

tox.ini

@@ -13,4 +13,6 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = {posargs}
 
 [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 doc/source doc/build/html