build: pin python to 3.12 (not 3.13) due to outdated dependencies

[youngjoon-lee]

Recently, the actions/setup-python@v5 has started installing Python 3.13 instead of 3.12. Ideally our code should work with 3.13, but installing dependencies is failing.

The first error is:

      ../scipy/meson.build:159:9: ERROR: Dependency "OpenBLAS" not found, tried pkgconfig and cmake

https://github.com/logos-co/nomos-specs/actions/runs/13521718813/job/37782294686#step:4:103
This can be resolved by upgrading scipy to the latest (1.15.2). I tested it in my local.

However, I got another error when installing poseidon-hash==0.1.4.

Collecting numba<0.60,>=0.55 (from galois~=0.3.6->poseidon-hash==0.1.4->-r requirements.txt (line 14))
...
      RuntimeError: Cannot install on Python version 3.13.2; only versions >=3.9,<3.13 are supported.

numba and galois have been upgraded for Python 3.13, but there's no update on poseidon-hash yet.

Until we have an updated version of poseidon-hash, we still should use Python 3.12.