Remove Cython < 3 specific tweaks and enforce Cython>3

[misl6]

• Updated the build process in .github/workflows/deploy.yml to use the build package for creating source distributions instead of directly invoking setuptools and Cython.
• Changed the build system requirements in pyproject.toml to require cython>3 for better compatibility with Cython 3.
• Simplified the dependency installation in .github/workflows/python-package.yml by removing unnecessary installation of cython and setuptools for the test environment.
• Updated the class name mangling in pyobjus/pyobjus.pyx to use _ObjcClass__cls_storage for Cython 3 compatibility, replacing the previous logic for handling different Cython versions.