python3Packages.mailman-fix: drop isPy36 reference

Python 3.6 was dropped a while ago.

pkgs/development/python-modules/mailcap-fix/default.nix

···
 { lib
 , buildPythonPackage
 , fetchPypi
-, isPy36
 }:
 
 buildPythonPackage rec {
   pname = "mailcap-fix";
   version = "1.0.1";
-  disabled = isPy36; # this fix is merged into python 3.6
+  format = "setuptools";
 
   src = fetchPypi {
     inherit pname version;