Don't execute setup.py directly, always use pip

Since Python 3.12, virtual environments don't have setuptools,
instead, pip will install it to an isolated build environment.

smoke/venv.sh

@@ -50,16 +50,6 @@ python -c 'import sys; print(sys.version)' | head -n1 | grep $VERSION
 
 # install packages with pip
 python -m pip install pytest
-if [ "$CYTHON" == "true" ]; then
-    if [ "$VERSION" == "3.12" ]; then
-        # Cython support for 3.12.0a1+ is not yet released
-        python -m pip install https://github.com/cython/cython/archive/master.tar.gz --install-option="--no-cython-compile"
-    else
-        # We try to fetch a wheel only and if that fails, we disable compilation
-        # Useful for fresh CPythons, where wheels are not yet ready, but Cython defaults to compiling
-        python -m pip install Cython --only-binary :all: || python -m pip install Cython --install-option="--no-cython-compile"
-    fi
-fi
 
 # run tests
 cat > test_foo.py << EOF
@@ -80,6 +70,12 @@ def two():
     return add(a, b)
 EOF
 
+cat > pyproject.toml << EOF
+[build-system]
+requires = ["setuptools", "cython"]
+build-backend = "setuptools.build_meta"
+EOF
+
 cat > setup.py << EOF
 from setuptools import setup
 from Cython.Build import cythonize
@@ -87,7 +83,7 @@ from Cython.Build import cythonize
 setup(ext_modules = cythonize('module.pyx'))
 EOF
 
-python setup.py build_ext --inplace
+python -m pip install .
 
 python -c 'import module; print(module.two())' | grep '^2$'
 fi