Fix pytest deprecation

tests/test_config.py

@@ -2,6 +2,7 @@
 import configparser as stdlib_configparser
 import io
 import pickle
+import warnings
 
 import pytest
 
@@ -20,7 +21,8 @@ def _create_test_config():
 
 def test_api_deprecation_warning():
     # accessing through configdefaults.config is the new best practice
-    with pytest.warns(None):
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
         root = configdefaults.config
         assert isinstance(str(root), str)