fix: fix imports when darts is not available (#16)

Author: lsorber

src/conformal_tights/_darts_forecaster.py

@@ -35,8 +35,8 @@
             _LikelihoodMixin,
         )
     except ImportError:
-        FUTURE_LAGS_TYPE = None
-        LAGS_TYPE = None
+        FUTURE_LAGS_TYPE = int
+        LAGS_TYPE = list[int]
 
         class TimeSeries: ...