Future proofing use of the six python version constants (#4238)

* Future proofing use of the six python version constants

After reading [1], noticed that cryptography uses a lot of if six.PY3
blocks. The issue with this is that whenever Python 4 is released,
this code in the else block will be executed even though it was
only intended for Python 2.

[1] http://astrofrog.github.io/blog/2016/01/12/stop-writing-python-4-incompatible-code/

Signed-off-by: Eric Brown <browne@vmware.com>

* Use not PY2 instead
3 files changed