Support Free Threading in Python 3.13

[ronaldoussoren]

Is your feature request related to a problem? Please describe.
Python 3.13 introduces an (experimental) free threading option (aka "NO_GIL"). PyObjC should support this feature.

Describe the solution you'd like

TL/DR: This is a lot more work than "build extension modules using a free threaded build"

Adding this requires a number of changes to PyObjC and the build process:

• Migrate all extension modules to 2 phase init
• Check all framework extension modules for possible threading issues and enable free threading for them
• Add fine grained locking in pyobjc-core, and then enable free threading
• Update the testing harness to also test a free threading build for 3.13 (both architectures)
• [ ]Update the distribution building script to also build free threaded extension modules
• Add documentation about support for free threading
• Add test scripts that stress tests the free threading support and run those with TSAN enabled

The hard part is updating pyobjc-core, that code relies on module global state that's protected by the GIL. A somewhat easy option is to switch to a "global pyobjc lock" in the short term and slowly peel of bits that can use more fine grained locking.