[Pkg-mailman-hackers] Bug#891881: mailman3-suite: On system using exim (no postfix install) the list create script tries to create postfix aliases

[Markus Gschwendt]

Package: mailman3
Version: 3.1.1
System: debian stretch-backports

This is a system with exim4 as local mailserver
On creating a new mailinglist i get following error in the logs:

```
File "/usr/lib/python3.5/wsgiref/handlers.py", line 137, in run
    self.result = application(self.environ, self.start_response)
  File "/usr/lib/python3/dist-
packages/mailman/database/transaction.py", line 50, in wrapper
    rtn = function(*args, **kws)
  File "/usr/lib/python3/dist-packages/mailman/rest/wsgiapp.py", line
214, in __call__
    return super().__call__(environ, start_response)
  File "falcon/api.py", line 215, in falcon.api.API.__call__
(falcon/api.c:2571)
  File "falcon/api.py", line 189, in falcon.api.API.__call__
(falcon/api.c:2190)
  File "/usr/lib/python3/dist-packages/mailman/rest/lists.py", line
245, in on_post
    mlist = create_list(**validator(request))
  File "/usr/lib/python3/dist-packages/mailman/app/lifecycle.py", line
100, in create_list
    call_name(config.mta.incoming).create(mlist)
  File "/usr/lib/python3/dist-packages/mailman/mta/postfix.py", line
66, in create
    self.regenerate()
  File "/usr/lib/python3/dist-packages/mailman/mta/postfix.py", line
103, in regenerate
    raise RuntimeError(NL.join(errors))
RuntimeError: command failure: /usr/sbin/postmap
/var/lib/mailman3/data/postfix_lmtp, 127, Key has expired
command failure: /usr/sbin/postmap
/var/lib/mailman3/data/postfix_domains, 127, Key has expired
Mar 01 21:34:02 2018 (11537) 127.0.0.1 - - "POST /3.0/lists HTTP/1.1"
500 59
```

This means that it tries to create postfix aliases but postfix is not
installed. /usr/sbin/postmap does not exist.

A workaround is to create that file (empty) with execute permissions.
A proper fix would be that
/usr/lib/python3/dist-packages/mailman/mta/postfix.py
will not be called in the first place.