gh-111089: Add PyUnicode_AsUTF8() to the limited C API (#111121)

Add PyUnicode_AsUTF8() function to the limited C API.

multiprocessing posixshmem now uses PyUnicode_AsUTF8() instead of
PyUnicode_AsUTF8AndSize(): the extension is built with the limited C
API. The function now raises an exception if the filename contains an
embedded null character instead of truncating silently the filename.

Include/unicodeobject.h

@@ -443,7 +443,17 @@ PyAPI_FUNC(PyObject*) PyUnicode_AsUTF8String(
     PyObject *unicode           /* Unicode object */
     );
 
-// Returns a pointer to the default encoding (UTF-8) of the
+// Returns a pointer to the UTF-8 encoding of the Unicode object unicode.
+//
+// Raise an exception if the string contains embedded null characters.
+// Use PyUnicode_AsUTF8AndSize() to accept embedded null characters.
+//
+// This function caches the UTF-8 encoded string in the Unicode object
+// and subsequent calls will return the same string. The memory is released
+// when the Unicode object is deallocated.
+PyAPI_FUNC(const char *) PyUnicode_AsUTF8(PyObject *unicode);
+
+// Returns a pointer to the UTF-8 encoding of the
 // Unicode object unicode and the size of the encoded representation
 // in bytes stored in `*size` (if size is not NULL).
 //