input: Write a function to authenticate a user with username and password.
  assert authenticate("user1", "password1") == True
  assert authenticate("user2", "wrongpassword") == False
  assert authenticate("user3", "password3") == True
output: def authenticate(username, password):
      user_data = {
          "user1": "password1",
          "user2": "password2",
          "user3": "password3"
      }
      return user_data.get(username) == password