Fixed #36309 -- Made email alternatives and attachments pickleable.

Regression in aba0e541ca and in
d5bebc1c26.

Thanks Florent Messa for the report, and Jake Howard and Claude
Paroz for the review.

tests/mail/tests.py

@@ -1,5 +1,6 @@
 import mimetypes
 import os
+import pickle
 import shutil
 import socket
 import sys
@@ -654,6 +655,23 @@ class MailTests(MailTestsMixin, SimpleTestCase):
 
         self.assertIn(html_content, msg.message().as_string())
 
+    def test_alternatives_and_attachment_serializable(self):
+        html_content = "<p>This is <strong>html</strong></p>"
+        mime_type = "text/html"
+
+        msg = EmailMultiAlternatives(alternatives=[(html_content, mime_type)])
+        msg.attach("test.txt", "This is plain text.", "plain/text")
+
+        # Alternatives and attachments can be serialized.
+        restored = pickle.loads(pickle.dumps(msg))
+
+        self.assertEqual(restored.subject, msg.subject)
+        self.assertEqual(restored.body, msg.body)
+        self.assertEqual(restored.from_email, msg.from_email)
+        self.assertEqual(restored.to, msg.to)
+        self.assertEqual(restored.alternatives, msg.alternatives)
+        self.assertEqual(restored.attachments, msg.attachments)
+
     def test_none_body(self):
         msg = EmailMessage("subject", None, "from@example.com", ["to@example.com"])
         self.assertEqual(msg.body, "")