security/py-keyczar: update to 0.715

Overview of Changes:

• Change version numbering to match PEP-0440
• Unpadding happens after MAC validation to prevent padding oracle attacks
• Added Encoding options to all crypto operations to select whether or not not encoding should be used.
• Python version setup improved for PyPi.
• AttachedSign with empty nonce now correctly functions.

While here limit python version to 2.x, because it doesn't build with 3.x

PR: 204576
Submitted by: douglas@douglasthrift.net (maintainer)