Replace deprecated pkg_resources with packaging in setup.py

[jasonwbarnett]

Summary

setup.py uses the deprecated pkg_resources module (from setuptools) to validate the installed Cython version when building from source. pkg_resources emits DeprecationWarning: pkg_resources is deprecated as an API on import, is slated for removal, and is unavailable in some modern setuptools configurations — which can make source builds warn or fail.

This ports the Cython version check to the packaging library:

# before
import pkg_resources
cython_dep = pkg_resources.Requirement.parse(CYTHON_DEPENDENCY)
if Cython.__version__ not in cython_dep:

# after
from packaging.requirements import Requirement
cython_dep = Requirement(CYTHON_DEPENDENCY)
if not cython_dep.specifier.contains(Cython.__version__, prereleases=True):

• Adds packaging>=20 to [build-system].requires so it is available at build time.
• pkg_resources.Requirement.__contains__ used prereleases=True internally, so passing prereleases=True preserves the original behavior exactly (including accepting in-range prereleases such as 3.3.0b1).

This mirrors the same fix already made in the sister project uvloop (MagicStack/uvloop@b377b7c).

Relationship to #1314

#1314 addresses the same deprecation by removing the Cython version check entirely. This PR is an alternative that keeps the check while dropping the deprecated dependency. Happy to defer to whichever direction you prefer.

Verification

• python -m py_compile setup.py passes; no pkg_resources references remain in the tree.
• Confirmed packaging.requirements.Requirement("Cython(>=3.2.1,<4.0.0)") parses the parenthesized PEP 508 form, and that .specifier.contains(v, prereleases=True) matches the old pkg_resources behavior across in-range, out-of-range, and prerelease versions.

Closes #1337