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

Fixes issue #6089.

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"))