I tried the doing it the way you did and got the same result. After checking with "dir()" I found a "commit" method. I tried running it and it worked.
['IUserManager', '__builtins__', 'a', 'abort', 'commit', 'config', 'getUtility', 'l', 'm', 'now', 'um', 'user_manager', 'x']
<bound method SABaseDatabase.commit of <mailman.database.postgresql.PostgreSQLDatabase object at 0x7efbf969c9e8>>
Maybe it was there because I'm using PostgreSQL but it worked for me.