sysrestore: Iterate over a list of dict keys

In Python 3, dict.keys() returns a key view. It is not safe to delete
dict keys when iterating over this view.

Convert the keys to list first.

Part of the work for https://fedorahosted.org/freeipa/ticket/4985

Reviewed-By: Martin Basti <mbasti@redhat.com>

ipapython/sysrestore.py

@@ -347,7 +347,7 @@ class StateFile:
         """
         root_logger.debug("Saving StateFile to '%s'", self._path)
 
-        for module in self.modules.keys():
+        for module in list(self.modules.keys()):
             if len(self.modules[module]) == 0:
                 del self.modules[module]