Python on Darwin: Don't depend on glibcLocales

glibcLocales isn't supported on Darwin any more, and as such it should not be used as a dependency.

Not sure if this is the best way to fix it; perhaps the package should be made optional in top-level instead (https://github.com/NixOS/nixpkgs/blob/93fd8fb3a54b21e836b5e06acff0ddb1dbc20d4b/pkgs/top-level/all-packages.nix#L5004 but I'm not sure how to make a derivation optional), or perhaps it should create an empty dir structure on Darwin.

anyway this removes some duplication, so it's good to apply in any case :grin: