sysutils/py-salt: Update to 2017.7.3

This is a bug fix release for the 2017.7.x branch. Changes this release:

https://docs.saltstack.com/en/latest/topics/releases/2017.7.3.html

PR: 225983
Submitted by: ohauer
Approved by: Christer Edwards (maintainer)
MFH: 2018Q1