Add stub for sys.gettrace

Author: msimacek

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/SysModuleBuiltins.java

@@ -668,4 +668,12 @@ boolean doGeneric() {
         }
     }
 
+    @Builtin(name = "gettrace")
+    @GenerateNodeFactory
+    abstract static class GetTrace extends PythonBuiltinNode {
+        @Specialization
+        static Object gettrace() {
+            return PNone.NONE;
+        }
+    }
 }