Fix python3_enable to return true

Recent regression spotted by Dmitry Tantsur.
DevStack dropped Py2 support but the now-unused-in-devstack
python3_enable got its result nastily inversed.

Change-Id: I4b37cc847a24705c4955cec2e6e45f0514705f1b
diff --git a/inc/python b/inc/python
index 727d52c..f98d28d 100644
--- a/inc/python
+++ b/inc/python
@@ -446,7 +446,7 @@
 # Report whether python 3 should be used
 # TODO(frickler): drop this once all legacy uses are removed
 function python3_enabled {
-    return 1
+    return 0
 }
 
 # Provide requested python version and sets PYTHON variable