Bad error message for list creation fail
If you try to create a list with an invalid name !!! you get the following error message:
HTTP Error 400: b'Invalid list name: !!!'
the b' part shouldn't be showing; there's something a bit off about the string display here.
This bug might be beginner friendly: the problem is that b'' is a string type indicator that shouldn't be showing. grepping for "Invalid list name" should help you pinpoint where the string comes from -- likely it's a mailman core error message that we aren't handling perfectly.