CPython now works w/ wasmtime 18 under preview2

While it took some upstream fixes as well as changes in CPython itself, wasmtime 18 now passes the test suite when --wasi preview2 is specified. This has all been backported through 3.11, although it isn’t available in an official release yet.

FYI support for WASI SDK 21 is not done yet. While it now compiles, test_importlib fails. I suspect it’s a test issue and not a semantic problem.