input: Write a python function to check if a string is a valid JSON.
  assert is_valid_json('{"name": "John", "age": 30, "city": "New York"}')==True
  assert is_valid_json('{"name": "John", "age": "30", "city": "New York"}')==True
  assert is_valid_json('{"name": "John", "age": 30, "city": "New York"')==False
  assert is_valid_json('Just a random string')==False
output: import json
  
  def is_valid_json(s):
      try:
          json.loads(s)
          return True
      except ValueError:
          return False