Add betterproto.Enum __copy__ and __deepcopy__ implementations (#566)

* Add betterproto.Enum __copy__ and __deepcopy__ implementations

betterproto.Enum is missing __copy__ and __deepcopy__ implementations, which were recently added to enum.Enum, see https://github.com/python/cpython/issues/106602
This fixes the bug where betterproto messages with Enums nested within cannot be copied via copy.deepcopy.

* Type hint on Enum.__copy__

Co-authored-by: James Hilton-Balfe <gobot1234yt@gmail.com>

* Type hint on Enum.__deepcopy__

Co-authored-by: James Hilton-Balfe <gobot1234yt@gmail.com>

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Co-authored-by: James Hilton-Balfe <gobot1234yt@gmail.com>