Fix udpate of acceptable aliases

Mailmailclient as of the current version (and the master) converts evertything to a flat uni-code string before submitting a REST request. This breaks the "Messages acceptance" config dialog which likes to submit the list of acceptable aliases as array (or list in Python talk). This commit allows for one-level lists (i.e. not lists-of-lists-of-lists but simply lists) to be submitted through the REST API.

This is one of a set to three commits to mailman-core, postorius and mailmanclient.