Fully support free threaded Python

[rogerbinns]

The GIL is currently relied upon. It will likely go away eventually.

https://py-free-threading.github.io/
https://docs.python.org/3.14/howto/free-threading-extensions.html

• Update unicode to say GIL not needed
• The utf8/offset mappers likely need critical sections
• Update apsw module to say GIL not needed

More in lower comment

[rogerbinns]

This is going to wait until at least 3.15

A good example of what is needed are the changes made to the _json C extension which processes the same Python data structures we do using the same APIs (dict, list, tuple, _FAST and similar upper-case macros.).

Much of it is undocumented such as the Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST macro, and I would want to use others like _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED.

It will be necessary to see what python/cpython#116738 does to make sure I get everything right.

[rogerbinns]

Something resembling a plan:

• wrapper method for all connection and cursor that CHECK_CLOSED, gets the connection mutex, calls original method, release mutex, return value. this can be autogenerated via macro for all except close
• using #define rename original method, and make wrapper be original method name calling redefined
• do something different about close
• don't support fork checker or shutdown
• check if global mutex needed, eg race between open and getting vfs, and unregistering vfs
• fixup all FAST methods on user supplied values