Server Upgrade: fix traceback caused by cidict

Traceback caused by recent py3 code migration.

https://fedorahosted.org/freeipa/ticket/5283

Reviewed-By: Petr Viktorin <pviktori@redhat.com>

ipaserver/install/schemaupdate.py

@@ -56,7 +56,7 @@ def _get_oid_dependency_order(schema, cls):
     # remove top_node from tree, it breaks ordering
     # we don't need this, tree from file is not consistent
     del tree[top_node]
-    unordered_oids = set(tree)
+    unordered_oids = set(tree.keys())
 
     # split into two groups, parents and child nodes, and iterate until
     # child nodes are not empty