add fixture to disable poping plot windows in tests

tests/conftest.py

@@ -0,0 +1,8 @@
+"""conftest.py contains fixtures that are automatically imported by pytest."""
+import pytest
+import matplotlib
+
+
+@pytest.fixture(autouse=True, scope='session')
+def set_matplotlib_backend():
+    matplotlib.use('Agg')  # to avoid opening windows during testing