mailmanPackages.python3: fix markdown support

Fixes the issue reported at
https://gitlab.com/mailman/mailman/-/issues/1137. The readme-renderer
package needs cmarkgfm for markdown support. Mailman's postorious needs
this to render the info field of mailing lists.

pkgs/servers/mail/mailman/python.nix

@@ -28,6 +28,10 @@ python3.override {
           hash = "sha256-WF3FFrnrBCphnvCjnD19Vf6BvbTfCaUsnN3g0Hvxqn0=";
         };
       });
+
+      readme-renderer = super.readme-renderer.overridePythonAttrs (_: {
+        propagatedBuildInputs = [ self.cmarkgfm ];
+      });
     })
 
     overlay;