Update test_cppext to stop invoking setuptools-based script as a CLI

This allows the "package" to be built using regular Python packaging
workflows instead of relying on `setuptools` being present in the
virtual environment.

Author: pradyunsg

Lib/test/cppextdata/setup.py

@@ -0,0 +1,43 @@
+# gh-91321: Build a basic C++ test extension to check that the Python C API is
+# compatible with C++ and does not emit C++ compiler warnings.
+import os
+import sys
+from test import support
+
+from setuptools import setup, Extension
+
+
+MS_WINDOWS = (sys.platform == 'win32')
+
+
+SOURCE = support.findfile('_testcppext.cpp')
+if not MS_WINDOWS:
+    # C++ compiler flags for GCC and clang
+    CPPFLAGS = [
+        # gh-91321: The purpose of _testcppext extension is to check that building
+        # a C++ extension using the Python C API does not emit C++ compiler
+        # warnings
+        '-Werror',
+    ]
+else:
+    # Don't pass any compiler flag to MSVC
+    CPPFLAGS = []
+
+
+def main():
+    cppflags = list(CPPFLAGS)
+    std = os.environ["CPYTHON_TEST_CPP_STD"]
+    name = os.environ["CPYTHON_TEST_EXT_NAME"]
+
+    cppflags = [*CPPFLAGS, f'-std={std}']
+
+    cpp_ext = Extension(
+        name,
+        sources=[SOURCE],
+        language='c++',
+        extra_compile_args=cppflags)
+    setup(name='internal' + name, version='0.0', ext_modules=[cpp_ext])
+
+
+if __name__ == "__main__":
+    main()

Lib/test/test_cppext.py

@@ -1,6 +1,10 @@
 # gh-91321: Build a basic C++ test extension to check that the Python C API is
 # compatible with C++ and does not emit C++ compiler warnings.
 import os.path
+try:
+    import ssl
+except ImportError:
+    ssl = None
 import sys
 import unittest
 import subprocess
@@ -12,7 +16,7 @@
 MS_WINDOWS = (sys.platform == 'win32')
 
 
-SETUP_TESTCPPEXT = support.findfile('setup_testcppext.py')
+PKG_CPPEXTDATA = os.path.dirname(support.findfile('setup.py', subdir="cppextdata"))
 
 
 @support.requires_subprocess()
@@ -31,6 +35,8 @@ def test_build_cpp03(self):
     @unittest.skipIf(
         '-fsanitize' in (sysconfig.get_config_var('PY_CFLAGS') or ''),
         'test does not work with analyzing builds')
+    # the test uses pip which needs a TLS connection to PyPI
+    @unittest.skipIf(ssl is None, 'No ssl module')
     # the test uses venv+pip: skip if it's not available
     @support.requires_venv_with_pip()
     def check_build(self, std_cpp03, extension_name):
@@ -76,14 +82,9 @@ def run_cmd(operation, cmd):
                     self.fail(
                         f"{operation} failed with exit code {proc.returncode}")
 
-        # Build the C++ extension
+        # Build and install the C++ extension
         cmd = [python, '-X', 'dev',
-               SETUP_TESTCPPEXT, 'build_ext', '--verbose']
-        run_cmd('Build', cmd)
-
-        # Install the C++ extension
-        cmd = [python, '-X', 'dev',
-               SETUP_TESTCPPEXT, 'install']
+               '-m', 'pip', 'install', PKG_CPPEXTDATA]
         run_cmd('Install', cmd)
 
         # Do a reference run. Until we test that running python