Avoid copy on raw pubkey objects

These pubkey objects are immutable, as noted in
pyca/cryptography#9403, so it should be safe to just
assign.

pgpy/packet/packets.py

@@ -1578,7 +1578,7 @@ def _extract_pubkey(self, pk: PubKey) -> None:
             elif isinstance(self.keymaterial, (NativeEdDSAPub, NativeCFRGXPub)):
                 if not isinstance(pk.keymaterial, (NativeEdDSAPub, NativeCFRGXPub)):
                     raise TypeError(f"Expected CFRG public key, got {type(pk.keymaterial)} instead")
-                pk.keymaterial._raw_pubkey = copy.copy(self.keymaterial._raw_pubkey)
+                pk.keymaterial._raw_pubkey = self.keymaterial._raw_pubkey
 
         pk.update_hlen()