FWIW, I just installed 2.1.26 on a test system, and that command does not work for me:
$ mailman-config File "/usr/lib/mailman/bin/mailman-config", line 34 print "mail_group: %s" % ""mail", "postfix", "mailman", "nobody", "daemon"" ^ SyntaxError: invalid syntax
Both that line and the following (for cgi_group) are double-quoted:
print "cgi_group: %s" % ""apache"" ... configure_opts: "--with-python=/usr/bin/python2.7 --without-permcheck"
The issue is you're running configure --without-permcheck and you haven't specified --with-mail-gid and --with-cgi-gid.
If you don't specify those, configure looks for an existing group for mail-gid from "mailman other mail daemon" and for cgi-gid from "www www-data nobody". If you don't specify --without-permcheck, configure complains if it doesn't find a group in the system from the list, but if you do specify --without-permcheck and configure doesn't find a group it uses the whole "mailman other mail daemon" and/or "www www-data nobody" string which causes the issue you're seeing in mailman-config.
There will also be group mismatch errors in the configured wrappers in this case.
Hm, part of that was an artifact of running configure manually instead of using the SPEC file I usually use to build Mailman. With the latter and my "fix" I get the following:
$ mailman-config Configuration and build information for Mailman
Mailman version: 2.1.26 Build Date: Wed Feb 7 13:23:45 CET 2018
prefix: /usr/lib/mailman var_prefix: /var/lib/mailman mailman_user: mailman mailman_group: mailman mail_group: mail postfix mailman nobody daemon cgi_group: apache
configure_opts: "--prefix=/usr/lib/mailman --with-var-prefix=/var/lib/mailman --with-config-dir=/etc/mailman --with-lock-dir=/var/lock/mailman --with-log-dir=/var/log/mailman --with-pid-dir=/var/run/mailman --with-queue-dir=/var/spool/mailman --with-python=/usr/bin/python2.7 --with-mail-gid=mail postfix mailman nobody daemon --with-cgi-id=apache --with-cgi-gid=apache --with-mailhost=localhost.localdomain --with-urlhost=localhost.localdomain --without-permcheck"
So it's still using --without-permcheck, but the other options are there.
Sebastian Hagedorn - Weyertal 121, Zimmer 2.02 Regionales Rechenzentrum (RRZK) Universität zu Köln / Cologne University - Tel. +49-221-470-89578