gh-92584: test_cppext uses setuptools (#92639)

Rewrite test_cppext to run in a virtual environment and to build the
C++ extension with setuptools rather than distutils.

Lib/test/setup_testcppext.py

@@ -0,0 +1,42 @@
+# 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 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 = [
+        # Python currently targets C++11
+        '-std=c++11',
+        # 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',
+        # Warn on old-style cast (C cast) like: (PyObject*)op
+        '-Wold-style-cast',
+        # Warn when using NULL rather than _Py_NULL in static inline functions
+        '-Wzero-as-null-pointer-constant',
+    ]
+else:
+    # Don't pass any compiler flag to MSVC
+    CPPFLAGS = []
+
+
+def main():
+    cpp_ext = Extension(
+        '_testcppext',
+        sources=[SOURCE],
+        language='c++',
+        extra_compile_args=CPPFLAGS)
+    setup(name="_testcppext", ext_modules=[cpp_ext])
+
+
+if __name__ == "__main__":
+    main()