Fix C++ compiler warnings: "zero as null pointer constant"
(clang -Wzero-as-null-pointer-constant).
* Add the _Py_NULL macro used by static inline functions to use
nullptr in C++.
* Replace NULL with nullptr in _testcppext.cpp.
Fix C++ compiler warnings on cast macros, like _PyObject_CAST(), when
casting a constant expression to a non constant type: use
const_cast<> in C++.
* In C++, Py_SAFE_DOWNCAST() now uses static_cast<> rather than
reinterpret_cast<>.
* Add tests to the _testcppext C++ extension.
* test_cppext no longer captures stdout in verbose mode.
Build a basic C++ test extension to check that the Python C API is
compatible with C++ and does not emit C++ compiler warnings.
* Add Modules/_testcppext.cpp: C++ extension
* Add Lib/test/test_cppext.py: test building the C++ extension.
Victor Stinner
serge-sans-paille