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 python/cpython#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>

---------

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

src/betterproto/enum.py

@@ -156,6 +156,12 @@ def __delattr__(self, item: Any) -> Never:
             f"{self.__class__.__name__} Cannot delete a member's attributes."
         )
 
+    def __copy__(self) -> Self:
+        return self
+
+    def __deepcopy__(self, memo: Any) -> Self:
+        return self
+
     @classmethod
     def try_value(cls, value: int = 0) -> Self:
         """Return the value which corresponds to the value.