GeezerVison

I think this is probably not be the right way to implement this check? Access from different threads could happen regardless of free-threading builds.

I think the concept the extension module might want to enforce is something like getting the current thread id (we have a python c api for this IIRC) upon first "use" of the object, saving that in the object state, and checking that current thread id == that thread id for subsequent uses.

first use could be construction... but unless the zstandard C API requires that, it may be better to consider first use the first time that ZStdCompressor is actually used by a C API. This allows for the pattern of:

A compressor or decompressor is created by one thread - and added to a thread pool or work queue to be actually used and exhausted - exclusively - in another thread.