don't suggest backporting PRs against refs other than master

tools/backport_pr.py

@@ -142,6 +142,8 b' def should_backport(labels=None, milestone=None):'
         if not pr['merged']:
             print ("Marked PR closed without merge: %i" % pr['number'])
             continue
+        if pr['base']['ref'] != 'master':
+            continue
         should_backport.add(pr['number'])
     return should_backport