[Stable/1.6-Only] Setting docs job for stable/1.6

Patch sets branch override on the doc building job and constrains
the sphinx version used for doc building

The previous minimum version of Sphinx allowed for incompatible
extensions in some edge cases. Incrementing and capping the minimum version allowed
and propagating this fix downstream addresses the issue.

Related-Bug: #1980465

Signed-off-by: Jiri Podivin <jpodivin@redhat.com>
Change-Id: I31c92a77450a57ee0c112a6a515311fcb07bacfa

.zuul.yaml

@@ -108,6 +108,7 @@
               - openstack-tox-cover
               - validations-libs-functional
         - openstack-tox-docs: &tripleo-docs
+            <<: *tox_override_checkout
             files:
               - ^doc/.*
               - ^README.rst

doc/requirements.txt

@@ -1,5 +1,5 @@
 # this is required for the docs build jobs
-sphinx>=1.8.0,!=2.1.0 # BSD
+sphinx>=2.2.0,<4.0.0!=2.1.0 # BSD
 openstackdocstheme>=1.30.0 # Apache-2.0
 doc8>=0.8.0 # Apache-2.0
 sphinxcontrib-apidoc>=0.2.0  # BSD

tox.ini

@@ -30,6 +30,7 @@ deps =
     -r {toxinidir}/test-requirements.txt
     -r {toxinidir}/doc/requirements.txt
 whitelist_externals = bash
+basepython = python3.8
 
 [testenv:bindep]
 # Do not install any requirements. We want this to be fast and work even if
@@ -112,4 +113,3 @@ commands =
 # Settings for doc8:
 extensions = .rst
 ignore = D001
-