WeakSet contains the __reduce__ method, but it isn't pickleable (and never was), because the pickle state contains the value of the __dict__ dict attribute which contains a reference to unpickleable local function _remove().

>>> pickle.dumps(weakref.WeakSet())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: Can't pickle local object 'WeakSet.__init__.<locals>._remove'

I wondering whether WeakSet should be made pickleable or the __reduce__ method should be removed. __reduce__() can be used also for copying, but  there are no tests for this feature.