Merge pull request scikit-learn#3196 from andrewclegg/master

Fix for Python 3.x race condition causing "RuntimeError: dictionary changed size during iteration"

Author: jnothman

sklearn/utils/testing.py

@@ -572,9 +572,9 @@ def func(*args, **kwargs):
 
 
 def clean_warning_registry():
-    """Safe way to reset warniings """
+    """Safe way to reset warnings """
     warnings.resetwarnings()
     reg = "__warningregistry__"
-    for mod in sys.modules.values():
+    for mod in sys.modules.copy().values():
         if hasattr(mod, reg):
             getattr(mod, reg).clear()