`iter_data_dirs` doesn't consider all site dirs

[flaviut]

For background, I'm working on NixOS, which has a really long $XDG_DATA_DIRS that includes about a dozen /nix/store/... paths, as well as

• /data/user/.nix-profile/share
• /nix/profile/share
• /data/user/.local/state/nix/profile/share
• /etc/profiles/per-user/user/share
• /nix/var/nix/profiles/default/share
• /run/current-system/sw/share

With that in mind, iter_data_dirs() and related functions are defined as

def iter_data_dirs():
    yield self.user_data_dir
    yield self.site_data_dir

When it probably should be this, in order to capture all the data directories rather than just the first one:

def iter_data_dirs():
    yield self.user_data_dir
    yield from self._site_data_dirs