Handle non-ascii characters in Mailman list members

Fix #872.

We use ('ascii', 'replace') rather than, say, utf-8, because it looks
like Mailman might not reliably accept utf-8, and it's not a big deal
if we lose some characters in names anyway.

(cherry picked from commit 8b4803ffc963c4b62519c4dd3b1d356932f903c3)