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.

/!\/!\/!\
Notice that I voluntarily added the doc directory even if no docs
are generated here because zuul will try to pull this requirements from
there first and the contained requirements are needed for reno but AFAIK
the releasenotes dir is ignored by zuul. c.f [4] for further details.
/!\/!\/!\

[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: I5cd2351ae4c813fac9a1909d518f8fa637807aaa

doc/requirements.txt

@@ -0,0 +1,3 @@
+sphinx>=2.0.0,!=2.1.0 # BSD
+openstackdocstheme>=2.2.1 # Apache-2.0
+reno>=3.1.0 # Apache-2.0

test-requirements.txt

@@ -1,9 +1,6 @@
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
-sphinx>=2.0.0,!=2.1.0 # BSD
 pre-commit==1.13.0 # MIT License
 hacking!=0.13.0,<0.14,>=0.12.0 # Apache-2.0
 bashate>=0.5.1  # Apache-2.0
-openstackdocstheme>=2.2.1 # Apache-2.0
-reno>=3.1.0 # Apache-2.0

tox.ini

@@ -44,6 +44,8 @@ commands =
     python -m pre_commit run -a
 
 [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