Fix some type problems

freqtrade/exchange/binance.py

@@ -57,7 +57,7 @@ class Binance(Exchange):
             (side == "buy" and stop_loss < float(order['info']['stopPrice']))
         )
 
-    def get_tickers(self, symbols: List[str] = None, cached: bool = False) -> Dict:
+    def get_tickers(self, symbols: Optional[List[str]] = None, cached: bool = False) -> Dict:
         tickers = super().get_tickers(symbols=symbols, cached=cached)
         if self.trading_mode == TradingMode.FUTURES:
             # Binance's future result has no bid/ask values.
@@ -95,7 +95,7 @@ class Binance(Exchange):
     async def _async_get_historic_ohlcv(self, pair: str, timeframe: str,
                                         since_ms: int, candle_type: CandleType,
                                         is_new_pair: bool = False, raise_: bool = False,
-                                        until_ms: int = None
+                                        until_ms: Optional[int] = None
                                         ) -> Tuple[str, str, str, List]:
         """
         Overwrite to introduce "fast new pair" functionality by detecting the pair's listing date