add cdef for pi
add cdef for pi

pi is no longer imported from numpy so must add it here

zasdfgbnm/qutip,cgranade/qutip,qutip/qutip,anubhavvardhan/qutip,cgranade/qutip,anubhavvardhan/qutip,zasdfgbnm/qutip,qutip/qutip