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.