Bump sdk-python pin to pick up Strands MCP client fix

Picks up c84320c6 on the strands branch, which rewrites the MCP
populate_cache and call_tool activity to use ClientSession directly
instead of MCPClient.start/stop. The old code created a background
event loop on one thread and closed it from another, which deadlocked
mcp_test on Python 3.10 once trio_asyncio's policy was installed at
import time by tests/trio_async/workflow_test.py.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: brianstrauch