refactor: clean up imports per PEP 8

Remove unused os and _clean_dataframe imports, group by stdlib/third-party/local.

tradingagents/dataflows/y_finance.py

@@ -1,10 +1,16 @@
-from typing import Annotated
 from datetime import datetime
+from typing import Annotated
+
 from dateutil.relativedelta import relativedelta
 import pandas as pd
 import yfinance as yf
-import os
+
-from .stockstats_utils import StockstatsUtils, _clean_dataframe, yf_retry, load_ohlcv, filter_financials_by_date
+from .stockstats_utils import (
+    StockstatsUtils,
+    filter_financials_by_date,
+    load_ohlcv,
+    yf_retry,
+)
 
 def get_YFin_data_online(
     symbol: Annotated[str, "ticker symbol of the company"],