py/modbuiltins: Fix getattr to work with class raising AttributeError.

Fixes issue #6089.

py/modbuiltins.c

@@ -558,7 +558,11 @@ MP_DEFINE_CONST_FUN_OBJ_KW(mp_builtin_sorted_obj, 1, mp_builtin_sorted);
 static inline mp_obj_t mp_load_attr_default(mp_obj_t base, qstr attr, mp_obj_t defval) {
     mp_obj_t dest[2];
     // use load_method, raising or not raising exception
-    ((defval == MP_OBJ_NULL) ? mp_load_method : mp_load_method_maybe)(base, attr, dest);
+    if (defval == MP_OBJ_NULL) {
+        mp_load_method(base, attr, dest);
+    } else {
+        mp_load_method_protected(base, attr, dest, false);
+    }
     if (dest[0] == MP_OBJ_NULL) {
         return defval;
     } else if (dest[1] == MP_OBJ_NULL) {

tests/basics/builtin_getattr.py

@@ -16,3 +16,15 @@ print(getattr(a, "meth")(5))
 print(getattr(a, "_none_such", 123))
 print(getattr(list, "foo", 456))
 print(getattr(a, "va" + "r2"))
+
+# test a class that defines __getattr__ and may raise AttributeError
+class B:
+    def __getattr__(self, attr):
+        if attr == "a":
+            return attr
+        else:
+            raise AttributeError
+b = B()
+print(getattr(b, "a"))
+print(getattr(b, "a", "default"))
+print(getattr(b, "b", "default"))