[python] avoid `deepcopy`

[msyyc]

For

typespec/packages/http-client-python/generator/pygen/codegen/serializers/import_serializer.py

Line 79 in 266f99c

file_import_copy = deepcopy(self.file_import)

, since deepcopy is very cost operation, we may be able to optimize performance if we could remove the deepcopy operation.