Fix SF bug #516768 reported by Dave Wallace.

Replace use of PyMem_DEL() with PyObject_Del() on object dealloc
functions.  The use of PyMem_DEL() is incorrect for object
deallocation, because it only ever calls the low-level free().  If a
custom allocator like pymalloc is used, it needs to be called to free
the memory.

Also replace bare malloc() and realloc() with PyMem_Malloc() and
PyMem_Realloc().  I think this isn't a strict bug fix, but a would be
nice.  It guarantees that BTrees objects get their memory from the
same allocator as the Python core.