python-mailman-web: turn these Djando configuration files into a make-shift Python library

Suggested in https://github.com/NixOS/nixpkgs/pull/67951#issuecomment-530309702.

nixos/modules/services/mail/mailman.nix

@@ -62,6 +62,12 @@ let
     installPhase = "install -D ${djangoPyEnv}/bin/django-admin $out/bin/mailman-django-admin";
   };
 
+  mailmanWeb = pkgs.python3Packages.mailman-web.override {
+    serverEMail = cfg.siteOwner;
+    archiverKey = cfg.hyperkittyApiKey;
+    allowedHosts = cfg.webHosts;
+  };
+
 in {
 
   ###### interface
@@ -88,10 +94,7 @@ in {
 
       webRoot = mkOption {
         type = types.path;
-        default = pkgs.python3Packages.mailman-web.override { serverEMail = cfg.siteOwner;
-                                                              archiverKey = cfg.hyperkittyApiKey;
-                                                              allowedHosts = cfg.webHosts;
-                                                            };
+        default = "${mailmanWeb}/${pkgs.python3.sitePackages}";
         defaultText = "pkgs.python3Packages.mailman-web";
         description = ''
           The web root for the Hyperkity + Postorius apps provided by Mailman.

pkgs/servers/mail/mailman/web.nix

@@ -1,4 +1,4 @@
-{ stdenv
+{ stdenv, python, hyperkitty, postorius, buildPythonPackage
 , serverEMail ? "postmaster@example.org"
 , archiverKey ? "SecretArchiverAPIKey"
 , allowedHosts ? []
@@ -10,18 +10,28 @@ let
 
 in
 
-stdenv.mkDerivation {
+# We turn those Djando configuration files into a make-shift Python library so
+# that Nix users can use this package as a part of their buildInputs to import
+# the code. Also, this package implicitly provides an environment in which the
+# Django app can be run.
+
+buildPythonPackage {
   name = "mailman-web-0";
 
+  propagatedBuildInputs = [ hyperkitty postorius ];
+
   unpackPhase = ":";
+  buildPhase = ":";
+  setuptoolsCheckPhase = ":";
 
   installPhase = ''
-    install -D -m 444 ${./urls.py} $out/urls.py
-    install -D -m 444 ${./wsgi.py} $out/wsgi.py
-    substitute ${./settings.py} $out/settings.py \
+    d=$out/${python.sitePackages}
+    install -D -m 444 ${./urls.py} $d/urls.py
+    install -D -m 444 ${./wsgi.py} $d/wsgi.py
+    substitute ${./settings.py} $d/settings.py \
       --subst-var-by SERVER_EMAIL '${serverEMail}' \
       --subst-var-by ARCHIVER_KEY '${archiverKey}' \
       --subst-var-by ALLOWED_HOSTS '${allowedHostsString}'
-    chmod 444 $out/settings.py
+    chmod 444 $d/settings.py
   '';
 }