Fix return type annotation

tests/fixtures/http/_mock_server.py

@@ -8,7 +8,7 @@
 import re
 from collections.abc import Awaitable, Callable, Sequence
 from copy import copy
-from typing import Any
+from typing import TYPE_CHECKING, Any
 
 import attrs
 from aiohttp.connector import TCPConnector
@@ -17,6 +17,7 @@
 from aiohttp.web import Response
 from aiohttp.web_request import BaseRequest
 from aiohttp.web_response import json_response
+from typing_extensions import Self
 from yarl import URL
 
 _Response = (
@@ -100,3 +101,7 @@ async def _resolve_host(
                 ]
 
         return _TCPConnector
+
+    if TYPE_CHECKING:
+        # Parent class returns "BaseTestServer".
+        async def __aenter__(self) -> Self: ...