* 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 4caf5c2753)
* test_cppext now builds the C++ extension with setuptools.
* Add @test.support.requires_venv_with_pip.
(cherry picked from commit ca0cc9c433)
