Support Python 3.12

[pschanely]

Currently, CrossHair will fail with this error:

% crosshair -h                                                           
Traceback (most recent call last):
  File "/Users/phillipschanely/.pyenv/versions/3.12.0b2/bin/crosshair", line 5, in <module>
    from crosshair.main import main
  File "/Users/phillipschanely/proj/CrossHair/crosshair/main.py", line 60, in <module>
    from crosshair.pure_importer import prefer_pure_python_imports
  File "/Users/phillipschanely/proj/CrossHair/crosshair/pure_importer.py", line 4, in <module>
    from importlib.abc import Finder
ImportError: cannot import name 'Finder' from 'importlib.abc' (/Users/phillipschanely/.pyenv/versions/3.12.0b2/lib/python3.12/importlib/abc.py)

But, that's just scratching the surface. There are significant changes under-the-hood interpreter changes that I think CrossHair will need to account for.