Tracking free-threading support

[minrk]

I don't expect to need any updates for free-threading support, but folks can't install Jupyter Server or JupyterLab with Python 3.13t due to two transitive dependencies:

• rpds-py, which uses PyO3 (this causes e.g. import jupyterlab to segfault because import rpds segfaults); pyO3 tracking issue: Tracking issue for no-gil/freethreaded work PyO3/pyo3#4265
• argon2-cffi-backend, which is waiting for a CFFI release to build with (likely 1.18): Free-threaded builds fail hynek/argon2-cffi-bindings#67

When I make an env working around these two issues (arm mac):

mamba create -n 313t python-freethreading pyzmq
mamba activate 313t
python3 -m pip install git+https://github.com/minrk/argon2-cffi-bindings@nogil-builds jupyterlab 'jsonschema<4.18'

(argon2 branch builds with HEAD of cffi, jsonschema is pinned to the last release before the rpds rust dependency), everything seems to work fine. That reinforces my suspicion that there won't be anything for us to do, but this is a tracking issue folks can link to, at least.

[bollwyvl]

Looks like there are still a few missing pieces for argon2-cffi. What about, even temporarily on a pre-release, moving that dep (back) to an [extra] (as it the PEP 508 way to ask for it hasn't been fully resolved yet) and falling back to a next-best stdlib hash function?