input: Write a python function to serialize a class instance into a JSON string.
  assert serialize_class(Person('Jane', 20)) == '{"name": "Jane", "age": 20}'
  assert serialize_class(Person('John', 30)) == '{"name": "John", "age": 30}'
  assert serialize_class(Person('Doe', 40)) == '{"name": "Doe", "age": 40}'
output: import json
  
  class Person:
      def __init__(self, name, age):
          self.name = name
          self.age = age
  
  def serialize_class(instance):
      return json.dumps(instance.__dict__)