Fix compilation warning when building against python 3
Fix compilation warning when building against python 3

tobgu/pyrsistent,jkbjh/pyrsistent,tobgu/pyrsistent,jkbjh/pyrsistent,Futrell/pyrsistent,jml/pyrsistent,jkbjh/pyrsistent,jml/pyrsistent,tobgu/pyrsistent,Futrell/pyrsistent,Futrell/pyrsistent,jml/pyrsistent