compat - 3.8 typing

tests/test_epoch.py:5: in <module>
    from pydantic_extra_types import epoch
pydantic_extra_types/epoch.py:12: in <module>
    class _Base(datetime.datetime):
pydantic_extra_types/epoch.py:27: in _Base
    cls, source: type[Any], handler: Callable[[Any], CoreSchema]
E   TypeError: 'type' object is not subscriptable

pydantic_extra_types/epoch.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import datetime
 from typing import Any, Callable, Optional