Python Client import_table Should not also Close the Writer

[nbauernfeind]

This fixes last night's nightly-ci failure. Note that the failure is difficult to reproduce as there is some racing involved. However, do note that DoPut requires the stream to be cancelled before it is considered complete. Since there are python tests that round-trip to ensure the data uploaded is readable we can be confident that the DoPut is being closed by the pyarrow library. (I did attempt to follow the code path through the cpp implementation, but it was hard for me to find the specific WriteTable call that is being invoked in this context.)

====================================================================== |  
  | ERROR [30.064s]: test_import_table_strings (test_session.SessionTestCase) |  
  | ---------------------------------------------------------------------- |  
  | Traceback (most recent call last): |  
  | File "/project/pydeephaven/_arrow_flight_service.py", line 34, in import_table |  
  | writer.close() |  
  | File "pyarrow/_flight.pyx", line 1166, in pyarrow._flight.MetadataRecordBatchWriter.close |  
  | File "pyarrow/_flight.pyx", line 62, in pyarrow._flight.check_flight_status |  
  | pyarrow._flight.FlightCancelledError: Flight cancelled call, with message: Received RST_STREAM with error code 8. gRPC client debug context: UNKNOWN:Error received from peer ipv4:172.21.0.2:10000 {created_time:"2023-11-06T06:25:08.49824529+00:00", grpc_status:1, grpc_message:"Received RST_STREAM with error code 8"}. Client context: OK |  
  |   |  
  | The above exception was the direct cause of the following exception: |  
  |   |  
  | Traceback (most recent call last): |  
  | File "/project/tests/test_session.py", line 177, in test_import_table_strings |  
  | new_table = self.session.import_table(pa_table) |  
  | File "/project/pydeephaven/session.py", line 486, in import_table |  
  | return self.flight_service.import_table(data=data) |  
  | File "/project/pydeephaven/_arrow_flight_service.py", line 39, in import_table |  
  | raise DHError("failed to create a Deephaven table from Arrow data.") from e |  
  | pydeephaven.dherror.DHError: failed to create a Deephaven table from Arrow data. |  
 ```

[jmao-denver]

LGTM