Python version compatibility

[j-c-cook]

Python 3.9 does not reach end of life for another year. The main branch currently has functionality that was not introduced until PEP 622, which was associated with Python 3.10.

The main branch of python-can supports Python 3.8+.

So there's a couple issues here:

1. Tests should build with all current active Python versions (Python 3.9+),
2. The pyproject.toml file should accurately describe the version, along with documentation in the README.

Traceback (most recent call last):
    ...
    match self.state:
          ^
SyntaxError: invalid syntax