gh-111089: Use PyUnicode_AsUTF8() in Argument Clinic (#111585)

Replace PyUnicode_AsUTF8AndSize() with PyUnicode_AsUTF8() to remove
the explicit check for embedded null characters.

The change avoids to have to include explicitly <string.h> to get the
strlen() function when using a recent version of the limited C API.

Author: vstinner