Do you know when this change will be deployed in Mailman3 or Python3? do i need to issue a bug report in order to speed up things?
This is the "missing hyphen in iso8859-7" issue, right? To answer the question literally, at present we don't want to put this in Mailman since the suggested code change is in Python.
A bug report to Mailman is unlikely to speed things up, since we want this in Python, not in Mailman (we'd have to vendor the stdlib email package). If you file a bug with Python, I don't know whether this would be picked up for Python 3.11 (already in beta), but since it's arguably a bug (cite IANA registration, where the standard MIME charset name is "ISO-8859-7", case-insensitive) it might be. In that case there would be a release in early October of this year. Since it's a bugfix it might be picked up for backport to any versions still in maintenance.
If not, it would likely be in Python 3.12, scheduled for release in early October of 2023.
Looking a little deeper at the ISO 8859 series of encodings in cpython/Lib/encodings, *all* of them have a line of the form "name='iso8859-$PARTNO'", not just iso8859_7.py. So I don't understand why this error hasn't happen for every ISO 8859 encoding a million times a day since 2000 or whenever outlook.com came online. They're not a small provider.
I will take another look at this but it may take some time.