input: Write a function to convert a duration in seconds to a time string in the format HH:MM:SS.
  assert to_time_string(32400) == "09:00:00"
  assert to_time_string(3661) == "01:01:01"
  assert to_time_string(59) == "00:00:59"
  assert to_time_string(3600) == "01:00:00"
output: def to_time_string(seconds):
      hours = seconds // 3600
      minutes = (seconds % 3600) // 60
      remaining_seconds = seconds % 60
      return f"{hours:02}:{minutes:02}:{remaining_seconds:02}"