gh-91321: Add _Py_NULL macro (#92253)

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.

Lib/test/test_cppext.py

@@ -29,6 +29,8 @@
         '-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