mail/mailman: update to 2.1.34 (bugfixes)

Changelog:
http://bazaar.launchpad.net/~mailman-coders/mailman/2.1/view/1859/NEWS#L8

(Note the ValueError fix was already in FreeBSD's 2.1.33_1 and

• on quarterly - the 2.1.30_5 port/package versions.)

Follow POLA:
No MFH requested, as 2020Q2 and head/ have diverged too far,
so let 2020Q3 pick up the change instead.