input: Write a python function to extract the value of a given key from a JSON string.
  assert extract_value('{"name": "John Smith", "age": 35}', "name") == "John Smith"
  assert extract_value('{"name": "John Smith", "age": 35}', "age") == 35
  assert extract_value('{"city": "New York", "country": "USA"}', "city") == "New York"
output: import json
  
  def extract_value(json_str, key):
      data = json.loads(json_str)
      return data.get(key)