python3Packages.pillow: update license
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:

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs