py/objtype: Use CPython compatible method name for sizeof.

Per https://docs.python.org/3/library/sys.html#sys.getsizeof:

getsizeof() calls the object’s __sizeof__ method. Previously, "getsizeof"
was used mostly to save on new qstr, as we don't really support calling
this method on arbitrary objects (so it was used only for reporting).
However, normalize it all now.