[Backport of r56382]
Avoid exception if there's a stray directory inside a Maildir folder.

The Maildir specification doesn't seem to say anything about this
situation, and it can happen if you're keeping a Maildir mailbox in
Subversion (.svn directories) or some similar system.  The patch just
ignores directories in the cur/, new/, tmp/ folders.
diff --git a/Lib/test/test_mailbox.py b/Lib/test/test_mailbox.py
index 18e6b67..57e44f4 100644
--- a/Lib/test/test_mailbox.py
+++ b/Lib/test/test_mailbox.py
@@ -685,7 +685,18 @@
         folder1_alias = box.get_folder('folder1')
         self.assert_(folder1_alias._factory is dummy_factory)
 
+    def test_directory_in_folder (self):
+        # Test that mailboxes still work if there's a stray extra directory
+        # in a folder.
+        for i in range(10):
+            self._box.add(mailbox.Message(_sample_message))
 
+        # Create a stray directory
+        os.mkdir(os.path.join(self._path, 'cur', 'stray-dir'))
+
+        # Check that looping still works with the directory present.
+        for msg in self._box:
+            pass
 
 class _TestMboxMMDF(TestMailbox):