py/deactivate vs slots: A slot variable could not be initialized at all

fe2219f (On deactivate release in-slots objects too) started to release
objects from slotted variables but was not careful enough while doing so: we
have to be careful while deleting as for unset variables it will raise
AttributeError:

    class C(object):
        __slots__ = ['aaa', 'unset']

        def __init__(self, aaa):
            self.aaa = 1

    c = C(111)
    del c.aaa
    del c.unset

    Traceback (most recent call last):
      File "y.py", line 9, in <module>
        del c.unset
    AttributeError: unset

Caught by preparing fix for #44 (comment)

C version already have this protection in the original fe2219f patch:

    +            if (PyObject_GenericSetAttr((PyObject *)self, name, NULL) < 0)
    +                /* delattr of non-set slot will raise AttributeError - we
    +                 * simply ignore. */
    +                PyErr_Clear();

persistent/persistence.py

@@ -429,7 +429,8 @@ def _p_invalidate_deactivate_helper(self, clear=True):
                 try:
                     getattr(type_, slotname).__delete__(self)
                 except AttributeError:
-                    pass
+                    # AttributeError means slot variable was not initialized at all -
+                    # - we can simply skip its deletion.
 
         # Implementation detail: deactivating/invalidating
         # updates the size of the cache (if we have one)

persistent/tests/test_persistence.py

@@ -1321,7 +1321,7 @@ def __init__(self):
 
     def test__p_invalidate_from_changed_w_slots(self):
         class Derived(self._getTargetClass()):
-            __slots__ = ('myattr1', 'myattr2')
+            __slots__ = ('myattr1', 'myattr2', 'unset')
             def __init__(self):
                 self.myattr1 = 'value1'
                 self.myattr2 = 'value2'