bpo-33254: do not return an empty list when asking for the contents of a namespace package (GH-6467)

diff --git a/Lib/importlib/abc.py b/Lib/importlib/abc.py
index d2f4509..dbdd5bf 100644
--- a/Lib/importlib/abc.py
+++ b/Lib/importlib/abc.py
@@ -381,8 +381,8 @@
 
     @abc.abstractmethod
     def contents(self):
-        """Return an iterator of strings over the contents of the package."""
-        return iter([])
+        """Return an iterable of strings over the contents of the package."""
+        return []
 
 
 _register(ResourceReader, machinery.SourceFileLoader)