Update sphinx jobs to use python3

The zuul sphinx jobs require python3 to run. Set the sphinx_python
variable so that it will.

Change-Id: Iee6d1f1003ba9464daffbc0009b82a21001e7739
Depends-On: I481e032834fdbf674157b2c9a8fa6f95fc570ddb
diff --git a/.zuul.yaml b/.zuul.yaml
index 041681a..a87c196 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -36,6 +36,8 @@
             irrelevant-files:
               - zuul/cmd/migrate.py
               - playbooks/zuul-migrate/.*
+            vars:
+              sphinx_python: python3
         - tox-cover:
             irrelevant-files:
               - zuul/cmd/migrate.py
@@ -53,6 +55,8 @@
             irrelevant-files:
               - zuul/cmd/migrate.py
               - playbooks/zuul-migrate/.*
+            vars:
+              sphinx_python: python3
         - tox-pep8
         - tox-py35:
             irrelevant-files:
@@ -61,5 +65,7 @@
         - zuul-stream-functional
     post:
       jobs:
-        - publish-openstack-sphinx-docs-infra
+        - publish-openstack-sphinx-docs-infra:
+            vars:
+              sphinx_python: python3
         - publish-openstack-python-branch-tarball