python3: fix weakref spewing exceptions during interp finalization

python3: fix weakref spewing exceptions during interp finalization

When py3 applications are exiting we often see errors similar to the
following:

Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fcb56b09400>
Traceback (most recent call last):
  File "/usr/lib64/python3.5/weakref.py", line 117, in remove
  TypeError: 'NoneType' object is not callable

After a quick search this was found to be a well reported issue
upstream and had an appropriate fix which is backported here.

Signed-off-by: Mark Asselstine <mark.asselstine@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>