List subclasses can be weakref'd (as mentioned by the docs), but their methods cannot be wrapped by WeakMethod, even though this makes sense semantically:
```
In [25]: class L(list): pass
In [26]: weakref.WeakMethod(L().append)
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-26-f6246b3b483e> in <module>()
----> 1 weakref.WeakMethod(L().append)
/usr/lib/python3.5/weakref.py in __new__(cls, meth, callback)
47 except AttributeError:
48 raise TypeError("argument should be a bound method, not {}"
---> 49 .format(type(meth))) from None
50 def _cb(arg):
51 # The self-weakref trick is needed to avoid creating a reference
TypeError: argument should be a bound method, not <class 'builtin_function_or_method'>
```
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