Add cmp function to packagequery module

cmp(a, b) returns
-1 if a < b
 0 if a == 0
 1 if a > b

This is needed since python3 has no cmp function anymore.

All credits for this go to Marco Strigl <mstrigl@suse.com> (see
PR#483 [1]).

[1] https://github.com/openSUSE/osc/pull/483

osc/util/packagequery.py

@@ -159,6 +159,11 @@ class PackageQueryResult:
             evr = epoch + ":" + evr 
         return evr 
 
+
+def cmp(a, b):
+    return (a > b) - (a < b)
+
+
 if __name__ == '__main__':
     import sys
     try: