Fix memleaks in pooptmodule.cut_text (ref #1914)

- we need to decrement the refcount after giving the tuple to the
  retlist in order to transfer ownership

The example script below will quickly take gigabytes of ram with the
old module, while the new will not take noticeably more memory.

The leak is not very visible on launch, because we “only” leaked each
tuple returned, and (int, int) is not heavy. However, after weeks of
use and many messages, the memory still isn’t freed and it shows.

import poopt
import gc

a = 'coucouco ' * 1000
for i in range(100000):
    if not (i % 10000):
        print(i)
    poopt.cut_text(a, 50)