refactor: change days type to Optional[int] in PreShiftConstraint and PostShiftConstraint

rota_generator/shifts.py

@@ -90,13 +90,13 @@ class NightConstraint(BaseShiftConstraint):
     options: Any = None
 
 class PreShiftConstraint(BaseShiftConstraint):
-    days: Any = None
+    days: Optional[int] = None
     ignore_shifts: List[str] = []
     exclude_days: List[str] = []
     allow_self: bool = True
 
 class PostShiftConstraint(BaseShiftConstraint):
-    days: Any = None
+    days: Optional[int] = None
     ignore_shifts: List[str]  = []
     exclude_days: List[str] = []
     allow_self: bool = True