input: Write a python function to get the phone number of a person from the dictionary.
  assert get_phone_number("John Smith") == "123-456-7890"
  assert get_phone_number("Jane Doe") == None
  assert get_phone_number("Alice Johnson") == None
output: def get_phone_number(name):
      phone_book = {
          "John Smith": "123-456-7890",
          "Mary Jane": "987-654-3210"
      }
      return phone_book.get(name)