Fix SQL syntax error when compare pair strings

First happens in Postgres

freqtrade/rpc/rpc.py

@@ -523,7 +523,7 @@ class RPC:
         # check if valid pair
 
         # check if pair already has an open pair
-        trade = Trade.get_trades([Trade.is_open.is_(True), Trade.pair.is_(pair)]).first()
+        trade = Trade.get_trades([Trade.is_open.is_(True), Trade.pair == pair]).first()
         if trade:
             raise RPCException(f'position for {pair} already open - id: {trade.id}')
 
@@ -532,7 +532,7 @@ class RPC:
 
         # execute buy
         if self._freqtrade.execute_buy(pair, stakeamount, price):
-            trade = Trade.get_trades([Trade.is_open.is_(True), Trade.pair.is_(pair)]).first()
+            trade = Trade.get_trades([Trade.is_open.is_(True), Trade.pair == pair]).first()
             return trade
         else:
             return None