Add missing import from b433bf5

Author: nocarryr

tests/test_filters.py

@@ -4,7 +4,7 @@
 from scipy import signal
 import pytest
 
-from lupy.filters import HS_COEFF, HP_COEFF
+from lupy.filters import HS_COEFF, HP_COEFF, FilterGroup
 
 
 @pytest.mark.parametrize('coeff', [HS_COEFF, HP_COEFF])