Norvig's originals and updates

This directory contains:

• norvig/: Norvig's lis.py unchanged, and lispy.py with minor changes to run on Python 3, and the lispytest.py custom test script;
• py3.9/: lis.py with type hints and a few minor edits—requires Python 3.9;
• py3.10/: lis.py with type hints, pattern matching, and minor edits—requires Python 3.10.

The py3.9/ and py3.10/ directories also have lis_test.py to run with pytest. These files include all the lis_tests suite from norvig/lispytest.py, and additional separate tests for each expression and special form handled by evaluate.