pyproject.toml: Add setuptools to dev group

Python 3.12 removed distutils from the standard library. Some Cython test
helpers import pyximport, which still imports distutils through setuptools'
compatibility shim during collection. Add setuptools to the dev test
dependencies so those tests can collect in cibuildwheel's test environment.

Author: sylwiaszunejko

pyproject.toml

@@ -49,6 +49,7 @@ dev = [
     "gevent",
     "eventlet>=0.33.3",
     "cython>=3.2",
+    "setuptools",
     "packaging>=25.0",
     "futurist",
     "pyyaml",