pythonPackages.Babel: Add comment about #75676

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

@@ -13,6 +13,10 @@ buildPythonPackage rec {
 
   checkInputs = [ pytest freezegun ];
 
+  # Note that a test will fail with an encoding error on Python 2 with Nix < 2.3
+  # due to https://github.com/NixOS/nixpkgs/pull/75676#issuecomment-579008837.
+  # TODO: Remove the above comment when we use a version that includes the fix
+  #       from https://github.com/python-babel/babel/pull/691
   doCheck = !stdenv.isDarwin;
 
   meta = with lib; {