Replace deprecated pkg_resources with packaging in setup.py

[jasonwbarnett]

What & why

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

Change

Port 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 in pyproject.toml 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 in the sister project MagicStack/uvloop@b377b7c.

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.

Related

• Upstream tracking issue: setup.py relies on deprecated pkg_resources MagicStack/asyncpg#1337
• Upstream PR Drop the Cython version check from setup.py MagicStack/asyncpg#1314 proposes an alternative (dropping the version check entirely). This PR keeps the check and removes the deprecated dependency, following the uvloop precedent.

[jasonwbarnett]

Superseded by the upstream PR MagicStack#1338 — this fix belongs against upstream rather than the company fork. Closing here.