fix: replace Python 3.10 union syntax in ToolContext for 3.9 compatibility

Author: vrtnis

src/agents/tool_context.py

@@ -1,6 +1,6 @@
 import asyncio
 from dataclasses import dataclass, field, fields
-from typing import Any
+from typing import Any, Optional
 
 from .run_context import RunContextWrapper, TContext
 
@@ -16,7 +16,7 @@ class ToolContext(RunContextWrapper[TContext]):
     tool_call_id: str = field(default_factory=_assert_must_pass_tool_call_id)
     """The ID of the tool call."""
 
-    _event_queue: asyncio.Queue[Any] | None = field(default=None, init=False, repr=False)
+    _event_queue: Optional[asyncio.Queue[Any]] = field(default=None, init=False, repr=False)
 
     @classmethod
     def from_agent_context(