Silence deprecation warning in test_mmap for ValueError.message.

Replacement is from 3.3, with 'p' added to make '...Regexp'.

Lib/test/test_mmap.py

@@ -470,14 +470,10 @@ class MmapTests(unittest.TestCase):
         f = open (TESTFN, 'w+b')
         f.close()
         with open(TESTFN, "rb") as f :
-            try:
-                m = mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READ)
-                m.close()
-                self.fail("should not have been able to mmap empty file")
-            except ValueError as e:
-                self.assertEqual(e.message, "cannot mmap an empty file")
-            except:
-                self.fail("unexpected exception: " + str(e))
+            self.assertRaisesRegexp(ValueError,
+                                   "cannot mmap an empty file",
+                                   mmap.mmap, f.fileno(), 0,
+                                   access=mmap.ACCESS_READ)
 
     def test_offset (self):
         f = open (TESTFN, 'w+b')