fix: add BaseExceptionGroup import for Python 3.10 compat in tests

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: g97iulio1609

tests/shared/test_exception_utils.py

@@ -3,12 +3,16 @@
 from __future__ import annotations
 
 import asyncio
+import sys
 
 import anyio
 import pytest
 
 from mcp.shared._exception_utils import collapse_exception_group, open_task_group
 
+if sys.version_info < (3, 11):
+    from exceptiongroup import BaseExceptionGroup
+
 
 # ---------------------------------------------------------------------------
 # collapse_exception_group() unit tests