v126: Mailman unable to view held messages for moderation

Symptoms

Attempting to view some held messages in Mailman returns the following page:

Additionally, the mailman error log returns a similar backtrace.

File "/usr/lib64/python3.6/email/message.py", line 578, in get_content_type
value = self.get('content-type', missing)
File "/usr/lib64/python3.6/email/message.py", line 471, in get
return self.policy.header_fetch_parse(k, v)
AttributeError: 'Message' object has no attribute 'policy'

Description

Since upgrading Mailman from Python 2 to Python 3, it has come to our attention that some pending or held moderation messages aren't viewable. The page returns the "Bug in the Mailman version" error message. We suspect this occurs due to the handling or parsing of message headers, but further review is needed.

We've opened an internal case for our development team to investigate this further. For reference, the case number is CPANEL-46525. Follow this article to receive an email notification when a solution is published in the product. 

Workaround

None available.