Close attachment stream at end of test

This avoids a warning with `-W default`:

ResourceWarning: unclosed file <_io.BufferedRandom name=14>

emails/tests/views_tests.py

@@ -628,7 +628,9 @@ def get_name_and_stream(self, message):
         """Get the first attachment's filename and data stream from a message."""
         for part in message.walk():
             if part.is_attachment():
-                return _get_attachment(part)
+                name, stream = _get_attachment(part)
+                self.addCleanup(stream.close)
+                return name, stream
         return None, None
 
     def test_short_attachment(self):