PyBytes_FromStringAndSize() fix

* PyBytes_FromStringAndSize() is an alias to
  PyString_FromStringAndSize() in Python 2, so it should
  not be redefined. On the contrary PyString_FromStringAndSize()
  does not exist in Python 3, and hence, to allow the
  Cython code to import it, without using it (when in
  Python 3), a fake declaration is created inside
  intbitset.h and intbitset_impl.c.
  (closes #9)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>