feat: add async context manager support to ClientTransport (linter)

Author: cchinchilla-dev

src/a2a/client/transports/base.py

@@ -1,15 +1,9 @@
-from __future__ import annotations
-
 from abc import ABC, abstractmethod
 from collections.abc import AsyncGenerator, Callable
-from typing import TYPE_CHECKING
+from types import TracebackType
 
 from typing_extensions import Self
 
-
-if TYPE_CHECKING:
-    from types import TracebackType
-
 from a2a.client.middleware import ClientCallContext
 from a2a.types import (
     AgentCard,