Unfortunately, Py_INCREF is sometimes used in an expression (followed by
a comma).  I wouldn't expect an assert to be valid there (and I'd want
to check ISO C to make sure it's portable, not just accepted by GCC).

I'd like if Py_INCREF and friends were made into static inline
functions, which *should* have identical performance (at least on GCC),
but that's a more significant change.