code: def test_astype_datetime(self, unit):
      ser = Series(iNaT, dtype=f"M8[{unit}]", index=range(5))
  
      ser = ser.astype("O")
      assert ser.dtype == np.object_
  
      ser = Series([datetime(2001, 1, 2, 0, 0)])
  
      ser = ser.astype("O")
      assert ser.dtype == np.object_
  
      ser = Series(
          [datetime(2001, 1, 2, 0, 0) for i in range(3)],
          dtype=f"M8[{unit}]",
      )
  
      ser[1] = np.nan
      assert ser.dtype == f"M8[{unit}]"
  
      ser = ser.astype("O")
      assert ser.dtype == np.object_
  
imports: from datetime import datetime, timedelta
  from importlib import reload
  import string
  import sys
  import numpy
  import pytest
  from pandas._libs.tslibs import iNaT
  import pandas.util._test_decorators
  from pandas import NA, Categorical, CategoricalDtype, DatetimeTZDtype, Index, Interval, NaT, Series, Timedelta, Timestamp, cut, date_range, to_datetime
  import pandas._testing
label_desc: Use image node_id 10 for calling the TestAstype obj's underlying member method code with example usage: obj.test_astype_datetime(unit) without return types