Update the code to better reflect recommended style:

Use != instead of <> since <> is documented as "obsolescent".
Use "is" and "is not" when comparing with None or type objects.
diff --git a/Lib/mailbox.py b/Lib/mailbox.py
index c92e145..53b2c19 100755
--- a/Lib/mailbox.py
+++ b/Lib/mailbox.py
@@ -30,7 +30,7 @@
             start = self.fp.tell()
             self._search_end()
             self.seekp = stop = self.fp.tell()
-            if start <> stop:
+            if start != stop:
                 break
         return rfc822.Message(_Subfile(self.fp, start, stop))