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: I233f7fe332a6f6c14ffb98081e3a9efc6a516722

doc/requirements.txt

@@ -0,0 +1,4 @@
+sphinx>=1.8.0,!=2.1.0 # BSD
+reno>=2.5.0 # Apache-2.0
+openstackdocstheme>=1.31.2 # Apache-2.0
+os-api-ref>=1.4.0 # Apache-2.0

test-requirements.txt

@@ -6,14 +6,10 @@ pylint>=2.4.0 # GPLv2
 coverage>=4.5.1 # Apache-2.0
 fixtures>=3.0.0 # Apache-2.0/BSD
 oslotest>=3.3.0 # Apache-2.0
-sphinx>=1.8.0,!=2.1.0 # BSD
 testrepository>=0.0.18 # Apache-2.0/BSD
 stestr>=2.0.0 # Apache-2.0
 testtools>=2.2.0 # MIT
 testresources>=2.0.1
 testscenarios>=0.5.0
-os-api-ref>=1.4.0 # Apache-2.0
-reno>=2.5.0 # Apache-2.0
-openstackdocstheme>=1.31.2 # Apache-2.0
 astroid>=2.2.0,<=3 # LGPLv2.1
 PyMySQL>=0.7.6 # MIT License

tox.ini

@@ -38,17 +38,21 @@ commands =
   pre-commit run --all-files --show-diff-on-failure
 
 [testenv:docs]
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf api-ref/build
 #  sphinx-build -W --keep-going -b html api-ref/source api-ref/build/html
   sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:api-ref]
+deps = {[testenv:docs]deps}
 commands =
   rm -rf api-ref/build
   sphinx-build -W -a -E --keep-going -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
 
 [testenv:releasenotes]
+deps = {[testenv:docs]deps}
 commands =
   sphinx-build -W -a -E -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html