Stop supporting Python 3.8

[gilch]

Python 3.8 reaches end of life this year. (October, I think.) That means I can upgrade Hissp's Python version.

I don't plan on making this change before the next release, provided said release happens before October.

Then there's the question of which minimum Python version to require next. The most conservative change would be 3.9, but I might be able to go a little farther without causing issues. I want Hissp to be usable on implementations other than CPython. (That means the at least the compiled output, but not necessarily the compiler itself.) Of those that aren't dead, they're usually a few versions behind, so I don't want to go all the way up to the current 3.13 preview (3.14 is in dev). 3.12 is CPython's current stable, but most alternative implementations are not that far yet, even important ones like PyPy and Graal Python, which are on 3.10 at the moment, I believe.

I think Nuitka and Brython are already on 3.12, so no problems there.

LTS Linux distros sometimes have pretty dated system Python. Even they wouldn't want to keep a Python version around beyond its end of life, although some very LTS distros might do their own security patches. I'm not going to worry about keeping 3.8 for this, but maybe there's a case for 3.9. I doubt Hissp has wide enough adoption for this to matter yet, but they can work around the issue by using an older Hissp release, or by sideloading a newer Python version.

That means I'll be targeting 3.10 next, at least. The new pattern matching statement should simplify the reader and compiler in a few places.