[Cython] Overflow semantics in pure Python mode

[Willem Broekema]

In Pure Python mode, code compiled (using annotations) to C should run
equivalent to running (ignoring annotations) in the Python
interpreter. That implies that a variable declared as cython.int may
not rely on overflow behavior, as Python would automatically make a
long object for such a result. And the same also holds for the other
numeric types, from char to longlong. Right?

- Willem