python3: Add makeshift django3 package till there's an official one.

pkgs/nixpkgs.nix

@@ -11,6 +11,7 @@ in {
       theme_flat-remix = callPackage ./flat-remix { };
       theme_sddm_midnight = callPackage ./sddm_midnight { };
       xdiskusage = callPackage ./xdiskusage { };
+      django3 = pkgs.python3Packages.callPackage ./django3 { };
     };
   };
 }