Add typing import to sampleStrategy

freqtrade/templates/sample_strategy.py

@@ -5,6 +5,7 @@
 import numpy as np  # noqa
 import pandas as pd  # noqa
 from pandas import DataFrame
+from typing import Optional, Union
 
 from freqtrade.strategy import (BooleanParameter, CategoricalParameter, DecimalParameter,
                                 IStrategy, IntParameter)