gh-121617: Include <string.h> for Py_CLEAR() macro (#144666)

Python.h now also includes <string.h> in the limited C API version 3.11
and newer to fix the Py_CLEAR() macro which uses memcpy().

Add a Py_CLEAR() test in test_cext.

Modify also _Py_TYPEOF to use C23 typeof() if available.