Update six requirement to version at which indexbytes was introduced

onepasswordpy uses `indexbytes`, which was only introduced in six 1.4.0 (cf https://bitbucket.org/gutworth/six/src/e468dba3c020c8cece8a9a7541ab9a018a8d01b4/CHANGES?at=default).
Merge pull request #3 from gardaud/patch-1

Update six requirement to version at which indexbytes was introduced