Amended pandas-dev#29016 according to comments

Switched from obj.__setattr__(...) to setattr(obj, ...)
python/mypy#7171 (comment)

Author: saska

pandas/tests/plotting/test_backend.py

@@ -9,7 +9,7 @@
 import pandas
 
 dummy_backend = types.ModuleType("pandas_dummy_backend")
-dummy_backend.__setattr__("plot", lambda *args, **kwargs: None)
+setattr(dummy_backend, "plot", lambda *args, **kwargs: None)
 
 
 @pytest.fixture