gh-91320: Fix more old-style cast warnings in C++ (#92247)

Use _Py_CAST(), _Py_STATIC_CAST() and _PyASCIIObject_CAST() in
static inline functions to fix C++ compiler warnings:
"use of old-style cast" (clang -Wold-style-cast).

test_cppext now builds the C++ test extension with -Wold-style-cast.

Lib/test/test_cppext.py

@@ -27,6 +27,8 @@ if not MS_WINDOWS:
         # 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',
     ]
 else:
     # Don't pass any compiler flag to MSVC