Use Cython 3 on Python 3.13+

[Mirochill]

Fixes #6.

Summary:

• keep the existing Cython==0.29.20 pin for Python versions before 3.13
• use Cython>=3.0.12,<4 on Python 3.13+, where the stdlib cgi module is no longer available
• keep setup.py and requirements.txt dependency constraints aligned

Validation:

• git diff --check HEAD~1..HEAD
• Static search confirmed the Cython constraints are limited to setup.py and requirements.txt, and Cython is imported only by jmpy/encrypt_py.py.
• Not run locally.