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

tests/test_defi_service.py

@@ -7,7 +7,6 @@ from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
 
-import defi_service
 from defi_service import (
     get_chain_tvls,
     get_dex_volumes,