input: Write a function to design a web server using the Flask framework in Python that will serve a JSON object containing the current temperature for a given city based on the city name provided in the request.
  assert get_temp(request.json={'city': 'London'})=={'temperature': 'some_temperature'}
  assert get_temp(request.json={'city': 'Paris'})=={'temperature': 'some_temperature'}
  assert get_temp(request.json={'city': 'New York'})=={'temperature': 'some_temperature'}
  assert get_temp(request.json={'city': 'Tokyo'})=={'temperature': 'some_temperature'}
  assert get_temp(request.json={'city': 'Sydney'})=={'temperature': 'some_temperature'}
output: def get_temp(request_json):
      city = request_json['city']
      temperature_data = {
          'London': '15',
          'Paris': '18',
          'New York': '20',
          'Tokyo': '22',
          'Sydney': '25'
      }
      return {'temperature': temperature_data.get(city, 'unknown')}