add Self to generated code imports

Author: benedikt-bartscher

betterproto2_compiler/src/betterproto2_compiler/templates/header.py.j2

@@ -18,7 +18,7 @@ import dateutil.parser
 import warnings
 from collections.abc import AsyncIterable, AsyncIterator, Iterable, Iterator
 import typing
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Self
 
 {% if output_file.settings.pydantic_dataclasses %}
 import pydantic