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.

Also removing useless requirements, and pulling them from the right
place.

[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

Depends-On: https://review.opendev.org/c/openstack/vitrage-tempest-plugin/+/769917
Change-Id: Ia841a2d4198b43bac46a5b5af01f52e1354eea4a

test-requirements.txt

@@ -3,7 +3,3 @@
 # process, which may cause wedges in the gate later.
 
 hacking>=3.0.1,<3.1.0 # Apache-2.0
-sphinx>=2.0.0,!=2.1.0 # BSD
-openstackdocstheme>=2.2.1 # Apache-2.0
-# releasenotes
-reno>=3.1.0 # Apache-2.0

tox.ini

@@ -35,7 +35,8 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
-deps = -r{toxinidir}/doc/requirements.txt
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/doc/requirements.txt
 whitelist_externals =
   rm
 commands =
@@ -43,6 +44,7 @@ commands =
   sphinx-build -E -W --keep-going -b html doc/source doc/build/html
 
 [testenv:releasenotes]
+deps = {[testenv:docs]deps}
 whitelist_externals =
   rm
 commands =