add publish-to-pypi-python3 project-template

Add a project template for projects that want to publish to PyPI using
python 3 to build their artifacts.

Change-Id: Ic8884892f70f27f0c015f8e01aae58cb203caf35
Depends-On: https://review.openstack.org/574374
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

zuul.d/project-templates.yaml

@@ -163,6 +163,30 @@
             dependencies:
               - release-openstack-python
 
+- project-template:
+    name: publish-to-pypi-python3
+    description: |
+      Publish a Python package to PyPI, then send release announcement
+      emails and propose updates to upper-constraints as needed.
+    pre-release:
+      jobs:
+        - release-openstack-python3
+        - announce-release:
+            dependencies:
+              - release-openstack-python3
+        - propose-update-constraints:
+            dependencies:
+              - release-openstack-python3
+    release:
+      jobs:
+        - release-openstack-python3
+        - announce-release:
+            dependencies:
+              - release-openstack-python3
+        - propose-update-constraints:
+            dependencies:
+              - release-openstack-python3
+
 - project-template:
     name: publish-to-pypi-neutron
     description: |