Merge email package 4.0 from the sandbox, including documentation, test cases,

and NEWS updates.

Lib/email/mime/nonmultipart.py

@@ -0,0 +1,26 @@
+# Copyright (C) 2002-2006 Python Software Foundation
+# Author: Barry Warsaw
+# Contact: email-sig@python.org
+
+"""Base class for MIME type messages that are not multipart."""
+
+__all__ = ['MIMENonMultipart']
+
+from email import errors
+from email.mime.base import MIMEBase
+
+
+
+class MIMENonMultipart(MIMEBase):
+    """Base class for MIME multipart/* type messages."""
+
+    __pychecker__ = 'unusednames=payload'
+
+    def attach(self, payload):
+        # The public API prohibits attaching multiple subparts to MIMEBase
+        # derived subtypes since none of them are, by definition, of content
+        # type multipart/*
+        raise errors.MultipartConversionError(
+            'Cannot attach additional subparts to non-multipart/*')
+
+    del __pychecker__