[fbsync] test_prototype_datasets_builtin: Properly close all streams (#7403)

Summary: Co-authored-by: Philip Meier <[email protected]>

Reviewed By: vmoens

Differential Revision: D44416630

fbshipit-source-id: efee1b0849c7e19ba1859f185bff421400046a12

Author: NicolasHug

test/test_prototype_datasets_builtin.py

@@ -123,7 +123,7 @@ def log_session_streams(self):
     def test_stream_closing(self, log_session_streams, dataset_mock, config):
         def make_msg_and_close(head):
             unclosed_streams = []
-            for stream in StreamWrapper.session_streams.keys():
+            for stream in list(StreamWrapper.session_streams.keys()):
                 unclosed_streams.append(repr(stream.file_obj))
                 stream.close()
             unclosed_streams = "\n".join(unclosed_streams)