*.c: Use the new `Py_TYPE' and `Py_SIZE' macros; define them if necessary.

*.c: Use the new `Py_TYPE' and `Py_SIZE' macros; define them if necessary.

Python 2.6 introduced macros to access object header fields, which is
handy because Python 3 changes the way object headers are
structured.  (I think the binary layout is unchanged, but the C-level
structuring is definitely different.)

This is the start of Python 3 porting work.