pythonPackages.demjson: bring back package attribute

Commit 2ca086da82
("pythonpackages.deform_bootstrap: Remove seemingly dead package")
removed it by mistake.

pkgs/top-level/python-packages.nix

@@ -4452,6 +4452,8 @@ in {
     };
   };
 
+  demjson = callPackage ../development/python-modules/demjson { };
+
   derpconf = self.buildPythonPackage rec {
     name = "derpconf-0.4.9";