Add missing import

Author: AdrienVannson

src/betterproto2_compiler/templates/header.py.j2

@@ -21,7 +21,7 @@ __all__ = (
 import builtins
 import datetime
 import warnings
-from collections.abc import AsyncIterable, AsyncIterator, Iterable
+from collections.abc import AsyncIterable, AsyncIterator, Iterable, Iterator
 import typing
 from typing import TYPE_CHECKING