python3Packages.pillow: update license

The url does not resolve anymore and after checking the current LICENSE
at https://github.com/python-pillow/Pillow/blob/master/LICENSE it states
that it is simply the HPND license.

> Like PIL, Pillow is licensed under the open source HPND License:

pkgs/development/python-modules/pillow/default.nix

@@ -23,7 +23,7 @@ import ./generic.nix (rec {
       supports many file formats, and provides powerful image
       processing and graphics capabilities.
     '';
-    license = "http://www.pythonware.com/products/pil/license.htm";
+    license = licenses.hpnd;
     maintainers = with maintainers; [ goibhniu prikhi SuperSandro2000 ];
   };
 } // args )