Attached patch modify the _struct module to use FASTCALL and Argument Clinic.

AC requires a summary line, so I duplicated the first sentence of iter_unpack() docstring:

iter_unpack(fmt, buffer, /)
    Return an iterator yielding tuples unpacked from the given bytes.
    
    Return an iterator yielding tuples unpacked from the given bytes
    source according to the format string, like a repeated invocation of
    unpack_from().  Requires that the bytes length be a multiple of the
    format struct size.