Have MimeWriter raise a DeprecationWarning as per PEP 4 and its documentation.
diff --git a/Misc/NEWS b/Misc/NEWS
index 1ce32ce..4ca7fa8 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -220,6 +220,8 @@
 Library
 -------
 
+- MimeWriter now raises a DeprecationWarning upon import.
+
 - tarfile.py: Improved unicode support. Unicode input names are now
   officially supported. Added "errors" argument to the TarFile class.