$25.$ Add Asserts to iso$\_8601$

The function iso$\_8601$ has its own preconditions. Add assert statements to enforce this precondition. Error messages are optional and are not required.

To test your assert statements, you will need to call the function iso$\_8601$ in the interactive shell. Calls like iso$\_8601(5)$ or iso$\_8601(\text{'}1$:$1$:$1\text{'})$ should crash with an AssertionError if you did it properly.

Check the Asserts

You may run this test multiple times.