chore: fix all ruff lint warnings

- Remove unused datetime imports from openbb_service, market_service,
  quantitative_service (now using obb_utils.days_ago)
- Remove unused variable 'maintains' in routes_sentiment
- Remove unused imports in test files
- Fix forward reference annotation in test helper

openbb_service.py

@@ -1,6 +1,5 @@
 import asyncio
 import logging
-from datetime import datetime, timedelta, timezone
 from typing import Any
 
 import yfinance as yf