rename symbol_is_pair to market_is_tradable

Make it part of the exchange class, so subclasses can override this

freqtrade/exchange/kraken.py

@@ -1,6 +1,6 @@
 """ Kraken exchange subclass """
 import logging
-from typing import Dict
+from typing import Any, Dict
 
 import ccxt
 
@@ -21,6 +21,16 @@ class Kraken(Exchange):
         "trades_pagination_arg": "since",
     }
 
+    def market_is_tradable(self, market: Dict[str, Any]) -> bool:
+        """
+        Check if the market symbol is tradable by Freqtrade.
+        Default checks + check if pair is darkpool pair.
+        """
+        parent_check = super().market_is_tradable(market)
+
+        return (parent_check and
+                market.get('darkpool', False) is False)
+
     @retrier
     def get_balances(self) -> dict:
         if self._config['dry_run']: