[gcdebug] Only ignore weakref.WeakKeyDictionary class on Python 2
From Python 3.5, this class fails isinstance check with the following
error:
::
cls = <class 'weakref.WeakKeyDictionary'>, instance = <functools._lru_list_elem object at 0x7f4331859a48>
def __instancecheck__(cls, instance):
"""Override for isinstance(instance, cls)."""
# Inline the cache checking
> subclass = instance.__class__
E AssertionError: [<class 'AttributeError'> in gc] 'functools._lru_list_elem' object has no attribute '__class__'
/usr/lib/python3.5/abc.py:181: AssertionError
I have no clue why this happens, but it makes
cubicweb.web.test.test_views.AutomaticWebTest.test_startup_views fail on
Python 3.5. So only consider this class for Python 2.