input: Write a python function to create a simple chatbot that responds to specific inputs.
  assert chatbot("hello") == "Hi there!"
  assert chatbot("how are you?") in ["I'm good!", "I'm doing alright!", "Not too bad!"]
  assert chatbot("goodbye") == "Goodbye!"
output: def chatbot(input):
      responses = {
          "hello": "Hi there!",
          "how are you?": ["I'm good!", "I'm doing alright!", "Not too bad!"],
          "goodbye": "Goodbye!"
      }
      if input in responses:
          if isinstance(responses[input], list):
              import random
              return random.choice(responses[input])
          else:
              return responses[input]
      return "Sorry, I don't understand."