Fix wrong type

freqtrade/exchange/exchange.py

@@ -946,7 +946,7 @@ class Exchange:
 
     # Order handling
 
-    def _lev_prep(self, pair: str, leverage: float, side: str):
+    def _lev_prep(self, pair: str, leverage: float, side: BuySell):
         if self.trading_mode != TradingMode.SPOT:
             self.set_margin_mode(pair, self.margin_mode)
             self._set_leverage(leverage, pair)
@@ -1068,7 +1068,7 @@ class Exchange:
 
     @retrier(retries=0)
     def stoploss(self, pair: str, amount: float, stop_price: float, order_types: Dict,
-                 side: str, leverage: float) -> Dict:
+                 side: BuySell, leverage: float) -> Dict:
         """
         creates a stoploss order.
         requires `_ft_has['stoploss_order_types']` to be set as a dict mapping limit and market