Avoid POSKeyError when commit occurs and we have savepoint that involves

Plone Site

When ZODB handles savepoint and we have changes in Plone Site at that
savepoint, it changes the `_p_estimated_size` attribute of Plone Site.
This is an assignment to one of the special persistency attributes
(identified by an _p_ name prefix); it should happen without access to
any other attributes of obj. But obj._tree is accessed in __setattr__ of
PloneSite class and results in a ZODB load which apparently fails.

See: plone/plone.restapi#1823 (comment)

Products/CMFPlone/Portal.py

@@ -61,7 +61,7 @@ def __getattr__(self, name):
 
     def __setattr__(self, name, obj):
         # handle re setting an item as an attribute
-        if self._tree is not None and name in self:
+        if not name.startswith("_") and self._tree is not None and name in self:
             del self[name]
             self[name] = obj
         else: