input: Write a python function to convert a JSON string to a JSON object and back to a JSON string.
  assert str_to_json('{"name": "John", "age": 30, "city": "New York"}') == '{"name": "John", "age": 30, "city": "New York"}'
  assert str_to_json('{"a": 1, "b": 2, "c": 3}') == '{"a": 1, "b": 2, "c": 3}'
  assert str_to_json('{"x": "value1", "y": "value2"}') == '{"x": "value1", "y": "value2"}'
output: def str_to_json(json_str):
      import json
      json_obj = json.loads(json_str)
      return json.dumps(json_obj)