Replace backticks with repr() or "%r"

From SF patch #852334.
diff --git a/Demo/metaclasses/Trace.py b/Demo/metaclasses/Trace.py
index 86e199d..ea12cd9 100644
--- a/Demo/metaclasses/Trace.py
+++ b/Demo/metaclasses/Trace.py
@@ -117,7 +117,7 @@
         def m2(self, y): return self.x + y
         __trace_output__ = sys.stdout
     class D(C):
-        def m2(self, y): print "D.m2(%s)" % `y`; return C.m2(self, y)
+        def m2(self, y): print "D.m2(%r)" % (y,); return C.m2(self, y)
         __trace_output__ = None
     x = C(4321)
     print x