CDNClient: Cope with longer manifest descriptions for some depots

Some depots now have a dict like {"public": {"gid": GID, ...}, ...}
where we would traditionally have seen a flat dict {"public": GID, ...}.
Not all depots have this: at the time of writing,
1628350 "Steam Linux Runtime - Sniper" does, but
1391110 "Steam Linux Runtime - Soldier" does not. Presumably this is a
change that is being rolled out gradually to let the Steam client give
better estimates of the size of a depot.

Cope with either format, by retrieving the gid field if the nested dict
is present.

Resolves: ValvePython#436
Signed-off-by: Simon McVittie <smcv@collabora.com>

steam/client/cdn.py

@@ -836,6 +836,12 @@ def async_fetch_manifest(
             else:
                 manifest_gid = depot_info.get('manifests', {}).get(branch)
 
+            if isinstance(manifest_gid, dict):
+                # For some depots, Steam has started returning a dict
+                # {"public": {"gid": GID, "size": ..., "download": ...}, ...}
+                # instead of a simple map {"public": GID, ...}
+                manifest_gid = manifest_gid['gid']
+
             if manifest_gid is not None:
                 tasks.append(
                     self.gpool.spawn(