py/objstr: Binary type of str/bytes for buffer protocol is 'B'.

The type is an unsigned 8-bit value, since bytes objects are exactly
that.  And it's also sensible for unicode strings to return unsigned
values when accessed in a byte-wise manner (CPython does not allow this).

py/objstr.c

@@ -1806,7 +1806,7 @@ mp_int_t mp_obj_str_get_buffer(mp_obj_t self_in, mp_buffer_info_t *bufinfo, mp_u
         GET_STR_DATA_LEN(self_in, str_data, str_len);
         bufinfo->buf = (void*)str_data;
         bufinfo->len = str_len;
-        bufinfo->typecode = 'b';
+        bufinfo->typecode = 'B'; // bytes should be unsigned, so should unicode byte-access
         return 0;
     } else {
         // can't write to a string