/usr/lib/tmpfiles.d/mailman3-web.conf is in mailman3-web 0+20170523-14.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | #Type Path Mode UID GID Age Argument
d /run/mailman3/web 0755 www-data www-data - -
|