rename symbol_is_pair to market_is_tradable

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

freqtrade/exchange/ftx.py

@@ -1,6 +1,6 @@
 """ FTX exchange subclass """
 import logging
-from typing import Dict
+from typing import Any, Dict
 
 from freqtrade.exchange import Exchange
 
@@ -12,3 +12,13 @@ class Ftx(Exchange):
     _ft_has: Dict = {
         "ohlcv_candle_limit": 1500,
     }
+
+    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('spot', False) is True)