Remove deprecated setup_requires in favor of PEP 517

[dkropachev]

Summary

Removes the deprecated setup_requires mechanism from setup.py in favor of the PEP 517 build system already configured in pyproject.toml.

Background

PEP 517 standardized how Python packages declare build dependencies via pyproject.toml's [build-system].requires. Before PEP 517, setup_requires in setup() was the way to pull in build-time dependencies like Cython. setuptools implemented this through fetch_build_eggs, which downloaded eggs at build time — bypassing pip's dependency resolver and causing various issues.

Starting with setuptools 70, using setup_requires emits a deprecation warning:

setuptools.installer and fetch_build_eggs are deprecated. Requirements should be satisfied by a PEP 517 installer.

Our pyproject.toml already declares Cython as a PEP 517 build dependency:

[build-system]
requires = ["setuptools>=70", "Cython"]

This means any PEP 517 compliant installer (pip, uv, build) will install Cython before setup.py runs, making setup_requires redundant.

Changes

• Remove setup_requires=['Cython>=3.0.11,<4'] from the setup() call in run_setup() — redundant with [build-system].requires and the source of the deprecation warning
• Remove pre_build_check() function — only existed to gate setup_requires; compiler availability is still handled gracefully by build_extensions.build_extension() which catches and reports compilation failures
• Remove egg_info bypass (try_cython &= 'egg_info' not in sys.argv) — existed to prevent setup_requires from triggering during pip's metadata phase, no longer relevant
• Remove CASS_DRIVER_ALLOWED_CYTHON_VERSION env var — only affected the setup_requires Cython version pin; users who need a specific Cython version can use UV_CONSTRAINT, PIP_CONSTRAINT, or pre-install it before building

What stays the same

• The build_extensions class and _setup_extensions() — these dynamically discover and compile Cython extensions at build time, which works fine under PEP 517
• The try_cython flag and CASS_DRIVER_NO_CYTHON env var — still controls whether Cython modules are built
• The dummy extension trick (ext_modules=[Extension('DUMMY', [])]) — still needed so setuptools invokes build_ext

Fixes #694

Test plan

• uv sync completes without PEP 517 deprecation warning
• python setup.py build_ext --inplace completes without deprecation warning
• Unit tests pass (590 passed, pre-existing failures unrelated)

[Lorak-mmk]

Great!