style: fix code formatting and linting issues

- Remove unused imports in stats_client.py and download_stats.py
- Fix import sorting in test files
- Remove unnecessary f-strings in server.py and demo script
- Clean up whitespace and formatting issues
- Ensure all files pass ruff and isort checks

Signed-off-by: longhao <hal.long@outlook.com>

pypi_query_mcp/core/stats_client.py

@@ -2,7 +2,6 @@
 
 import asyncio
 import logging
-from datetime import datetime, timedelta
 from typing import Any
 
 import httpx