pythonPackages.klein: don't mark as broken

The package was fixed for all systems in da4f4e183c1dde9c8481d4719df2c518ea9fa34a.

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

···
     homepage = "https://github.com/twisted/klein";
     license = licenses.mit;
     maintainers = with maintainers; [ exarkun ];
-    broken = (stdenv.isLinux && stdenv.isAarch64) || stdenv.isDarwin;
   };
 }