Use renamed template 'integrated-gate-py3'

'integrated-gate-py35' template is going to be
renamed to 'integrated-gate-py3' in https://review.openstack.org/#/c/626078/

Integrated jobs are running on Bionic now where python 3.6 is available.
Which means gate jobs in 'integrated-gate-py35' template are
running on python 3.6 not on 3.5 which makes this template name confusing.

depends on commit rename the 'integrated-gate-py35' to 'integrated-gate-py3'
so that it can convey that template will use available python 3 version
in used distro. For example: 3.5 in xenial and 3.6 in bionic.

This commit starts using the new template name so that old
template name can be removed.

Depends-On: https://review.openstack.org/#/c/626078/
Change-Id: Ic5264b3b79d0a4caaa7f466f8686d38f54fe1ffe
diff --git a/.zuul.yaml b/.zuul.yaml
index 93d1db5..ef9b0eb 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -449,7 +449,7 @@
     templates:
       - check-requirements
       - integrated-gate
-      - integrated-gate-py35
+      - integrated-gate-py3
       - openstack-cover-jobs
       - openstack-python-jobs
       - openstack-python35-jobs