CLN: fix mypy error pandas/tests/plotting/test_backend.py (pandas-dev#29016)

* CLN: fix mypy error pandas/tests/plotting/test_backend.py

GH28926, mypy didn't like setting non-existant attribute on module.
Dummy function, circumvented with __setattr__

Author: saska

pandas/tests/plotting/test_backend.py

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

setup.cfg

@@ -202,9 +202,6 @@ ignore_errors=True
 [mypy-pandas.tests.io.json.test_ujson]
 ignore_errors=True
 
-[mypy-pandas.tests.plotting.test_backend]
-ignore_errors=True
-
 [mypy-pandas.tests.series.test_constructors]
 ignore_errors=True