Free-threading support

[FFY00]

WIP, don't merge.

[FFY00]

The kwlist const change is not strictly required given how it is currently being used, but it's a good idea to avoid static variables when possible. This way the compiler will error out on bad changes, instead of potentially introducing bugs on free-threading builds.

The rest of the static variables are only used to construct str objects that shouldn't change, so that shouldn't introduce issues on free-threading builds. I don't think it's worth dropping the static modifier, as that would introduce performance hits, though very minor.

[FFY00]

From my review of the code, I didn't catch anything else that would be problematic on free-threaded builds — there's no global state other than the static variables mentioned above, and I didn't catch any other instances of unsafe borrowed reference usage.

I'll add some multi-threaded tests now.