Cleanup quality (#21493)

* Remove mentions of flake8/isort

* Clean up inits

* Deall with all other inits

* Last special rule for dummy files

utils/check_dummies.py

@@ -115,7 +115,6 @@ def create_dummy_files(backend_specific_objects=None):
     for backend, objects in backend_specific_objects.items():
         backend_name = "[" + ", ".join(f'"{b}"' for b in backend.split("_and_")) + "]"
         dummy_file = "# This file is autogenerated by the command `make fix-copies`, do not edit.\n"
-        dummy_file += "# flake8: noqa\n"
         dummy_file += "from ..utils import DummyObject, requires_backends\n\n"
         dummy_file += "\n".join([create_dummy_object(o, backend_name) for o in objects])
         dummy_files[backend] = dummy_file