Enable part of this code because Pyston support co_names now.

We have to support self.used. This part of code was disabled
because Pyston didn't support co_names. Now enable it. But Pyston's
bytecode is different than CPython bytecode. So we can't use the code
which rely on co_code part.

src/RestrictedPython/Eval.py

@@ -72,11 +72,11 @@ class RestrictionCapableEval:
         if self.ucode is None:
             # Use the standard compiler.
             co = compile(self.expr, '<string>', 'eval')
-            # Pyston change: return code directly
-            # if self.used is None:
-            #     # Examine the code object, discovering which names
-            #     # the expression needs.
-            #     names=list(co.co_names)
+            if self.used is None:
+                # Examine the code object, discovering which names
+                # the expression needs.
+                names=list(co.co_names)
+            # Pyston change: Pyston use different bytecode system than CPython
             #     used={}
             #     i=0
             #     code=co.co_code
@@ -92,6 +92,7 @@ class RestrictionCapableEval:
             #         elif c >= HAVE_ARGUMENT: i=i+3
             #         else: i=i+1
             #     self.used=tuple(used.keys())
+                self.used=tuple(names)
             self.ucode=co
 
     def eval(self, mapping):