Update test_heterogeneity_indices.py

Author: gnopik

tests/test_heterogeneity_indices.py

@@ -1,12 +1,12 @@
 import importlib
 
-import matplotlib
-matplotlib.use("Agg")
 import matplotlib.pyplot as plt
 import numpy as np
 import pandas as pd
 import pytest
 
+plt.switch_backend("Agg")
+
 hi = importlib.import_module("simdec.heterogeneity_indices")