chatbot_pt4.py suggestion

Hi,
This is not an issue, just a suggestion.  For chatbot_pt4.py, I would like to suggest an alternative solution for the function is_valid_input(user_input, valid_responses) like so:

```
def is_valid_input(user_input, valid_responses):
    if user_input in valid_responses:
        return True
    return False
```

Thank you for your attention.  I appreciate it.
Merilyn