Hey all! 👋

This aims to be a tracking issue for all work necessary to support the free-threaded build of Python 3.13. A very high-level list of steps is (more details to follow as I investigate more issues):

• Set up CI for free-threading
  • Test 3.13 free-threaded build on CI #8200
• Move away from APIs that use borrowed references under the free-threaded build
  • Move away from APIs that use borrowed references under the free-threaded build #8216
• Audit C extension module for thread-safety issues
  • Lock around usages of imaging memory arenas #8238
• Mark C extension modules as thread-safe with the Py_mod_gil slot (or PyUnstable_Module_SetGIL)
  • Move away from APIs that use borrowed references under the free-threaded build #8216
• Upload wheels for the free-threaded build. Other projects have started uploading nightlies for the free-threaded on the scientific-python-nightly-wheels index so that we can enable faster iteration, we can discuss whether we wanna do this for Pillow as well.
  • Build free-threaded wheels #8219
  • Request access for Pillow scientific-python/upload-nightly-action#84
  • Upload wheels to scientific-python-nightly-wheels index #8236
  • Skip QEMU-emulated wheels on workflow dispatch event #8243
  • Move uploading nightly wheels to different job #8244