Replace backticks with repr() or "%r"

From SF patch #852334.
diff --git a/Demo/scripts/pp.py b/Demo/scripts/pp.py
index 64e57ee..92a1104 100755
--- a/Demo/scripts/pp.py
+++ b/Demo/scripts/pp.py
@@ -125,6 +125,6 @@
 fp.flush()
 if DFLAG:
 	import pdb
-	pdb.run('execfile(' + `tfn` + ')')
+	pdb.run('execfile(%r)' % (tfn,))
 else:
 	execfile(tfn)