minor update to make __pow__ comply with standard signature
minor update to make __pow__ comply with standard signature

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