input: Write a python function to convert a date string in 'YYYY-MM-DD' format to ISO 8601 format using datetime module.
  assert convert_to_iso8601('2020-07-15') == '2020-07-15T00:00:00'
  assert convert_to_iso8601('2021-12-01') == '2021-12-01T00:00:00'
  assert convert_to_iso8601('1999-01-01') == '1999-01-01T00:00:00'
output: def convert_to_iso8601(date_string):
      from datetime import datetime
      date_obj = datetime.strptime(date_string, '%Y-%m-%d')
      return date_obj.isoformat()