Test also swift-dsvm-functional-py3

Swift keeps testing py2 but we should keep both in shape.

To fix stestr on py2:
Depends-on: https://review.opendev.org/715942

Change-Id: I616e39c64e22d467d7186dba98226cc5beef23ea

Author: yoctozepto

.zuul.yaml

@@ -602,9 +602,12 @@
             voting: false
         - swift-dsvm-functional:
             voting: false
-            irrelevant-files:
+            irrelevant-files: &dsvm-irrelevant-files
               - ^.*\.rst$
               - ^doc/.*$
+        - swift-dsvm-functional-py3:
+            voting: false
+            irrelevant-files: *dsvm-irrelevant-files
         - grenade-py3:
             irrelevant-files:
               - ^.*\.rst$