Move to Python 3.11

[slott56]

Start with testing in 3.8, 3.9, 3.10, and 3.11 environments in the tox.ini configuration.

Because of changes to typing module, there are two choices.

1. Set mypy to a specific version, like 3.8.
2. Introduce version-specific type-checking features. For example...

import sys
if sys.version_info < (3, 9):
    from typing import Dict
else:
    from typing import TypeAlias
    Dict: TypeAlias = dict

When version 3.8 is dropped from support, the Dict references are changed to dict and this version-specific import is removed.