math/py-keras: Update to 2.1.3

WHile here:

Allow concurrent installation.
Relax USES=python, Keras is compatible with Python 2.7-3.6.

Changes: https://github.com/keras-team/keras/releases/tag/2.1.3

PR: 225463
Submitted by: Neel Chauhan <neel@neelc.org> (maintainer)