One more. Note that Path class cannot be weakref'd, at least in Py 2.…

…7, due to string subclass.

plumbum/commands/modifiers.py

@@ -59,7 +59,7 @@ def returncode(self):
 
 
 class ExecutionModifier(object):
-    __slots__ = ()
+    __slots__ = ("__weakref__")
 
     def __repr__(self):
         """Automatically creates a representation for given subclass with slots.