If the python version of Persistent had __setstate__ called with a dictionary containing exactly two items, it would fail at idict.update(inst_dict) with 'ValueError: sequence of pairs expected' due to incorrect tuple unpacking. Do what the C version does and check for a tuple explicitly.

Fix traversal of the python picklecache ring when invalidating if the order doesn't exactly match the ring

- Denominate files to indicate ReST.

- Drop .bzrignore.

See http://bugs.python.org/issue16676