Pytype_GenericAlloc(): round up size so we zap all four bytes of the

Pytype_GenericAlloc(): round up size so we zap all four bytes of the
__dict__ slot for string subtypes.

subtype_dealloc(): properly use _PyObject_GetDictPtr() to get the
(potentially negative) dict offset.  Don't copy things into local
variables that are used only once.

type_new(): properly calculate a negative dict offset when tp_itemsize
is nonzero.  The __dict__ attribute, if present, is now a calculated
attribute rather than a structure member.