Drop pkg_resources from setup.py; retire setuptools<81 build pin

Replace the pkg_resources import (removed in setuptools>=81) with importlib.metadata
+ packaging.version (open-mmlab#3326/open-mmlab#3328). The wheel build no longer needs the
setuptools<81 workaround, so the GitHub Action and justfile build-wheel now install
plain setuptools (+ packaging).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: yuki-inaho

.github/workflows/build-cu121-wheels.yml

@@ -64,8 +64,8 @@ jobs:
       - name: Install torch (cu121) + build deps
         run: |
           set -eux
-          # setuptools<81 still ships pkg_resources, which mmcv's setup.py imports.
-          python${PYTHON_VERSION} -m pip install "setuptools<81" wheel ninja "numpy<2"
+          # setup.py no longer needs pkg_resources, so any setuptools works.
+          python${PYTHON_VERSION} -m pip install setuptools wheel ninja packaging "numpy<2"
           python${PYTHON_VERSION} -m pip install "torch==${TORCH_VERSION}" \
             --index-url "${CU_INDEX}"
 

justfile

@@ -56,7 +56,7 @@ build-wheel CC="8.6":
     if [ -f pyproject.toml ]; then mv pyproject.toml .pyproject.devenv.bak; fi
     uv run --no-project --python 3.10 \
         --with "torch=={{ TORCH_VERSION }}" --with "numpy<2" \
-        --with pip --with "setuptools<81" --with wheel --with ninja \
+        --with pip --with setuptools --with wheel --with ninja --with packaging \
         --index-strategy unsafe-best-match --extra-index-url "{{ CU_INDEX }}" \
         env MMCV_WITH_OPS=1 FORCE_CUDA=1 TORCH_CUDA_ARCH_LIST="{{ CC }}" \
         python -m pip wheel --no-build-isolation --no-deps -w "dist/cc_{{ CC }}" . -v ; \

setup.py

@@ -2,9 +2,26 @@
 import os
 import platform
 import re
-from pkg_resources import DistributionNotFound, get_distribution, parse_version
 from setuptools import find_packages, setup
 
+# Avoid the deprecated pkg_resources (removed in setuptools>=81). Use
+# importlib.metadata + packaging.version instead.
+from importlib.metadata import PackageNotFoundError
+from importlib.metadata import version as _pkg_version
+from packaging.version import parse as parse_version
+
+
+class DistributionNotFound(Exception):
+    """Raised by get_distribution() when a package is not installed."""
+
+
+def get_distribution(name):
+    """Probe whether ``name`` is installed; raise DistributionNotFound if not."""
+    try:
+        _pkg_version(name)
+    except PackageNotFoundError as exc:
+        raise DistributionNotFound(name) from exc
+
 EXT_TYPE = ''
 try:
     import torch