Add support for Python free-threaded build

[JCGoran]

Currently it just builds, there are a lot of test failures. The docs also do not build because jupyter and ipython depend on argon2-cffi, which does not support the free-threaded build yet.

Changes:

• bump pybind11 to 2.13
• bump Cython to at least 3.1
• add CMake support for detecting free-threaded builds
• add free-threaded build to CI
• update Dockerfile

References:

• Port for use with Python 3.13 free-threaded build? hynek/argon2-cffi#187
• https://cython.readthedocs.io/en/latest/src/userguide/freethreading.html
• https://docs.python.org/3.13/howto/free-threading-extensions.html
• https://pybind11.readthedocs.io/en/stable/advanced/misc.html#misc-free-threading
• https://github.com/pybind/pybind11/releases/tag/v2.13.0
• https://labs.quansight.org/blog/free-threaded-one-year-recap
• https://nanobind.readthedocs.io/en/latest/free_threaded.html
• https://cibuildwheel.pypa.io/en/stable/options/#enable
• https://docs.python.org/3.13/using/mac.html#installing-using-the-command-line
• https://stackoverflow.com/a/71697223

[mgeplf]

What's the plan for all the static variables? Is there a plan to encapsulate all the state that could be shared between threads?

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

✔️ 75dcd13 -> artifacts URL

[github-actions]

✔️ e3c9cc2 -> artifacts URL

[codecov]

Codecov Report

Attention: Patch coverage is 30.76923% with 9 lines in your changes missing coverage. Please review.

Project coverage is 68.45%. Comparing base (3b1cd44) to head (e3c9cc2).

Files with missing lines	Patch %	Lines
src/nrnpython/nrnpy_nrn.cpp	30.76%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3536      +/-   ##
==========================================
- Coverage   68.45%   68.45%   -0.01%     
==========================================
  Files         683      683              
  Lines      116617   116611       -6     
==========================================
- Hits        79835    79823      -12     
- Misses      36782    36788       +6     

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