code: def test_timezone_conversion() -> None:
      """
      Test that columns with timezones are converted to a string.
      """
      df = pd.DataFrame(
          {"dt": [datetime(2023, 1, 1, 0, 0, tzinfo=timezone.utc)]}
      )
      contents = df_to_excel(df)
      assert (
          pd.read_excel(contents)["dt"][0]
          == "2023-01-01 00:00:00+00:00"
      )
  
imports: from datetime import datetime, timezone
  import pandas
  from superset.utils.excel import df_to_excel
label_desc: Use image node_id 1 for calling a global function with example usage: test_timezone_conversion() without return types