python-PyICU: disable for darwin

The build errors out on macOS.

Changed files
+1 -1
pkgs
+1 -1
pkgs/top-level/python-packages.nix
···
homepage = https://pypi.python.org/pypi/PyICU/;
description = "Python extension wrapping the ICU C++ API";
license = licenses.mit;
-
platforms = platforms.all;
+
platforms = with platforms; allBut darwin;
maintainers = [ maintainers.rycee ];