gh-91321: Fix compatibility with C++ older than C++11 (#93784) (#93802)

* Fix the compatibility of the Python C API with C++ older than C++11.
* _Py_NULL is only defined as nullptr on C++11 and newer.

(cherry picked from commit 4caf5c2753f1aa28d6f4bc1aa377975fd2a62331)

* test_cppext now builds the C++ extension with setuptools.
* Add @test.support.requires_venv_with_pip.

(cherry picked from commit ca0cc9c433830e14714a5cc93fb4e7254da3dd76)