fix: add Python 3.10 compat import for BaseExceptionGroup

On Python < 3.11, BaseExceptionGroup is not a builtin and must be
imported from the exceptiongroup backport package (transitive dep
via anyio).

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

Author: g97iulio1609

src/mcp/shared/_exception_utils.py

@@ -14,11 +14,15 @@
 
 import asyncio
 import contextlib
+import sys
 from collections.abc import AsyncIterator
 
 import anyio
 import anyio.abc
 
+if sys.version_info < (3, 11):
+    from exceptiongroup import BaseExceptionGroup
+
 
 def collapse_exception_group(
     eg: BaseExceptionGroup,

src/mcp/shared/session.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import logging
+import sys
 from collections.abc import Callable
 from contextlib import AsyncExitStack
 from types import TracebackType
@@ -12,6 +13,9 @@
 from typing_extensions import Self
 
 from mcp.shared._exception_utils import collapse_exception_group
+
+if sys.version_info < (3, 11):
+    from exceptiongroup import BaseExceptionGroup
 from mcp.shared.exceptions import MCPError
 from mcp.shared.message import MessageMetadata, ServerMessageMetadata, SessionMessage
 from mcp.shared.response_router import ResponseRouter