Sync requirements with OS requirements

This essentially reverts Icd31caa93587614425036fc4c8d3cfd3a83b6b3c,
as the incompatibility with six 1.4.x has been fixed meanwhile.

Change-Id: I40c588ca005ea3a8b04e64cfe3b86bbed69ee0a0
Merge "Sync requirements with OS requirements"