All persistence margin tests pass

Flake8 compliant, passed mypy, ran isort .

freqtrade/enums/interestmode.py

@@ -1,16 +1,20 @@
-from enum import Enum, auto
 from decimal import Decimal
+from enum import Enum
+
 from freqtrade.exceptions import OperationalException
 
+
 one = Decimal(1.0)
 four = Decimal(4.0)
 twenty_four = Decimal(24.0)
 
 
 class InterestMode(Enum):
+    """Equations to calculate interest"""
 
     HOURSPERDAY = "HOURSPERDAY"
     HOURSPER4 = "HOURSPER4"  # Hours per 4 hour segment
+    NONE = "NONE"
 
     def __call__(self, *args, **kwargs):
 
@@ -21,4 +25,4 @@ class InterestMode(Enum):
         elif self.name == "HOURSPER4":
             return borrowed * rate * (1 + max(0, (hours-four)/four))
         else:
-            raise OperationalException(f"Leverage not available on this exchange with freqtrade")
+            raise OperationalException("Leverage not available on this exchange with freqtrade")