Define constraints file for docs and renos

Right now we are not using any constraints for docs and releasenotes builds.
This has resulted in docs job failures once Sphinx 7.2.0 has been released.

The patch will ensure that constraints are used an we should not face
simmilar issue again.

TOX_CONSTRAINTS_FILE is updated by Release bot once new branch is created,
so it should always track relevant constraints.

Some extra syntax-related changes can apply, since patch is being passed
through ConfigParser, that does not preserve comments and align indenting.


Change-Id: I7ad481f26903519c5f1d9ab0e075ef56d7e9f091

tox.ini

@@ -31,7 +31,9 @@ setenv =
     WORKING_DIR={toxinidir}
 
 [testenv:docs]
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+    -r{toxinidir}/doc/requirements.txt
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
 commands =
     bash -c "rm -rf doc/build"
     doc8 doc
@@ -45,17 +47,16 @@ commands =
     sphinx-build -W --keep-going -b latex doc/source doc/build/pdf
     make -C doc/build/pdf
 
-
 [doc8]
-# Settings for doc8:
 extensions = .rst
 
 [testenv:releasenotes]
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+    -r{toxinidir}/doc/requirements.txt
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
 commands =
     sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html
 
-# environment used by the -infra templated docs job
 [testenv:venv]
 commands =
     {posargs}
@@ -65,9 +66,6 @@ commands =
     bash -c "{toxinidir}/tests/common/test-pep8.sh"
 
 [flake8]
-# Ignores the following rules due to how ansible modules work in general
-#     F403 'from ansible.module_utils.basic import *' used;
-#          unable to detect undefined names
 ignore = F403
 
 [testenv:bashate]