Suppress matplotlib warnings on non-GUI use of show() (#5749)

Tests intentionally use a non-GUI matplotlib backend "agg".
Reduce clutter in the pytest output.

Author: pavoljuhas

pyproject.toml

@@ -3,3 +3,8 @@ line-length = 100
 target_version = ['py37', 'py38', 'py39']
 skip-string-normalization = true
 skip-magic-trailing-comma = true
+
+[tool.pytest.ini_options]
+filterwarnings = [
+    "ignore:Matplotlib is currently using agg:UserWarning",
+]