input: Write a python function to convert a date string to a datetime object.
  assert date_string_to_datetime("2023-10-05") == datetime.date(2023, 10, 5)
  assert date_string_to_datetime("1999-12-31") == datetime.date(1999, 12, 31)
  assert date_string_to_datetime("2000-01-01") == datetime.date(2000, 1, 1)
output: from datetime import datetime
  
  def date_string_to_datetime(date_string):
      return datetime.strptime(date_string, "%Y-%m-%d").date()