mailman-web: fix django version check removal

Recent versions had increased the amount of indentation, which stopped
this applying.  The next version will also change the case, so I've
adjusted in advance for that too.

pkgs/servers/mail/mailman/web.nix

@@ -16,7 +16,7 @@ buildPythonPackage rec {
     # Django is depended on transitively by hyperkitty and postorius,
     # and mailman_web has overly restrictive version bounds on it, so
     # let's remove it.
-    sed -i '/^  Django/d' setup.cfg
+    sed -i '/^[[:space:]]*django/Id' setup.cfg
 
     # Upstream seems to mostly target installing on top of existing
     # distributions, and uses a path appropriate for that, but we are