Add none check for unicode slicing, remove duplicate code, add test for non-ascii unicode strings and None

Add none check for unicode slicing, remove duplicate code, add test for non-ascii unicode strings and None

it should be enough to restart tracking the object at the end of tp_dealloc() only if there really is a base type that might rely on it

--HG--
extra : rebase_source : 8a20a41f35f9b5d1aaa2a916b52403bd0cb145fc

--HG--
extra : rebase_source : cf2a0d87f2f903179cc954ff092c79fef7b13a34