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 could meet the conditions leading to the bug, and
uniformization can't hurt. These changes will reduce the likelihood to face
pip resolver incompatibilities by splitting requirements.

[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-

Change-Id: I75450f05993a5e1fbebe647ffaab440a9a63bf95

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

@@ -1,7 +1,4 @@
 # 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
-openstackdocstheme>=2.2.1 # Apache-2.0
 doc8>=0.6.0  # Apache-2.0

tox.ini

@@ -20,6 +20,8 @@ commands = {posargs}
 commands = doc8 -v doc README.rst
 
 [testenv:docs]
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/doc/requirements.txt
 commands = docstheme-build-translated.sh
 
 [doc8]