py: Make MP_OBJ_NEW_SMALL_INT cast arg to mp_int_t itself.

Addresses issue #724.
diff --git a/py/objbool.c b/py/objbool.c
index 9bf30d4..72535b2 100644
--- a/py/objbool.c
+++ b/py/objbool.c
@@ -74,7 +74,7 @@
 
 STATIC mp_obj_t bool_binary_op(int op, mp_obj_t lhs_in, mp_obj_t rhs_in) {
     if (MP_BINARY_OP_OR <= op && op <= MP_BINARY_OP_NOT_EQUAL) {
-        return mp_binary_op(op, MP_OBJ_NEW_SMALL_INT((mp_int_t)mp_obj_is_true(lhs_in)), rhs_in);
+        return mp_binary_op(op, MP_OBJ_NEW_SMALL_INT(mp_obj_is_true(lhs_in)), rhs_in);
     }
     return MP_OBJ_NULL; // op not supported
 }