[MNT] Force headless Matplotlib backend in tests to prevent UI crashes

Author: Siddhazntx

tests/conftest.py

@@ -1,9 +1,12 @@
 import os
 import sys
 
+import matplotlib
 import numpy as np
 import pytest
 
+matplotlib.use("Agg")
+
 sys.path.insert(0, os.path.abspath(os.path.join(__file__, "../..")))  # isort:skip