apply 2to3

Index: utilities/prover9-mace4
--- utilities/prover9-mace4.orig
+++ utilities/prover9-mace4
@@ -23,12 +23,12 @@ import subprocess
 def code_to_message(program, code):
 
     if program == 'Mace4':
-        if code in mace4_exits.keys():
+        if code in list(mace4_exits.keys()):
             message = mace4_exits[code]
         else:
             message = 'unknown'
     elif program == 'Prover9':
-        if code in prover9_exits.keys():
+        if code in list(prover9_exits.keys()):
             message = prover9_exits[code]
         else:
             message = 'unknown'
