py: Add mp_raise_type helper macro and use it where appropriate.

This provides a more consistent C-level API to raise exceptions, ie moving
away from nlr_raise towards mp_raise_XXX.  It also reduces code size by a
small amount on some ports.

extmod/modurandom.c

@@ -154,7 +154,7 @@ STATIC mp_obj_t mod_urandom_choice(mp_obj_t seq) {
     if (len > 0) {
         return mp_obj_subscr(seq, mp_obj_new_int(yasmarang_randbelow(len)), MP_OBJ_SENTINEL);
     } else {
-        nlr_raise(mp_obj_new_exception(&mp_type_IndexError));
+        mp_raise_type(&mp_type_IndexError);
     }
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(mod_urandom_choice_obj, mod_urandom_choice);