code: def test_serde_time_unit() -> None:
      assert pickle.loads(
          pickle.dumps(
              pl.Series(
                  [
                      datetime(2022, 1, 1) + timedelta(days=1)
                      for _ in range(3)
                  ]
              ).cast(pl.Datetime("ns"))
          )
      ).dtype == pl.Datetime("ns")
  
imports: from __future__ import annotations
  import io
  import pickle
  from datetime import datetime, timedelta
  import pytest
  import polars
  from polars import StringCache
  from polars.testing import assert_frame_equal, assert_series_equal
label_desc: Use image node_id 4 for calling a global function with example usage: test_serde_time_unit() without return types