Use upper-constraints for docs jobs

Looks like there are some issues with too-new-Sphinx and the PDF docs
builds.

Also, get devstack installing on bionic again for our py2 func test job.

Change-Id: I633398054694fe6ba1e0de50278f274daf69fefd

.zuul.yaml

@@ -33,6 +33,10 @@
     description: |
       Run functional tests of python-swiftclient under Python 2
     vars:
+      devstack_localrc:
+        # devstack dropped support for bionic, but we want it for easier py2 support.
+        # Set this so we install anyway.
+        FORCE: "yes"
       tox_envlist: py2func
 
 - project:

tox.ini

@@ -65,7 +65,8 @@ commands = {[testenv:func]commands}
 [testenv:docs]
 basepython = python3
 usedevelop = False
-deps = -r{toxinidir}/doc/requirements.txt
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/doc/requirements.txt
 commands=
     sphinx-build -W -b html doc/source doc/build/html -W
 
@@ -99,7 +100,8 @@ commands = bindep test
 [testenv:releasenotes]
 basepython = python3
 usedevelop = False
-deps = -r{toxinidir}/doc/requirements.txt
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -W -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:pdf-docs]