Implement __deepcopy__ for Message (#339)

src/betterproto/__init__.py

@@ -8,6 +8,7 @@ import sys
 import typing
 from abc import ABC
 from base64 import b64decode, b64encode
+from copy import deepcopy
 from datetime import datetime, timedelta, timezone
 from dateutil.parser import isoparse
 from typing import (
@@ -717,6 +718,14 @@ class Message(ABC):
             for field_name in self._betterproto.meta_by_field_name
         )
 
+    def __deepcopy__(self: T, _: Any = {}) -> T:
+        kwargs = {}
+        for name in self._betterproto.sorted_field_names:
+            value = self.__raw_get(name)
+            if value is not PLACEHOLDER:
+                kwargs[name] = deepcopy(value)
+        return self.__class__(**kwargs)  # type: ignore
+
     @property
     def _betterproto(self) -> ProtoClassMetadata:
         """