Fix .filter-kwargs lookup crash during cached runs

[flaeppe]

When copying class methods via copy_method_to_another_class, build arguments from the signature (CallableType) instead of from FuncDef.arguments as FuncDef.arguments are not serialized by mypy. Currently, methods copied on cached rounds have no arguments, due to an AttributeError being raised here(given that method_node is built from cache):

django-stubs/mypy_django_plugin/lib/helpers.py

Lines 409 to 412 in 220f0e4

	try:
	original_arguments = method_node.arguments[1:]
	except AttributeError:
	original_arguments = []

Related issues

Fixes #827

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Note: I've run this commit with cache enabled on the project where I encountered the error seen in #827, without being able to recreate it.

[sobolevn]

Thanks! I have one concern 🙂

[sobolevn]

Looks good to me! Thank you!