Fix pickling for python < 3.4
Fix pickling for python < 3.4

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