Support for free-threaded Python

Hi everyone!

I'm opening an issue first of all to ask whether there's somebody that's already working on free-threading support. If not, I would like to help as much as I can. The steps are roughly the following:

• Set up CI for free-threading (Not sure if this is relevant here, since AFAIU brotli does not exhaustively test different Python versions)
• Audit C extension module for thread-safety issues
• Mark C extension module as thread-safe with PyUnstable_Module_SetGIL
• Build & upload nightly wheels for the free-threaded build (This includes some minor changes over at google/brotli-wheels, but those shouldn't be complicated at all)