https://github.com/saltstack/salt/pull/45687 and
https://github.com/saltstack/salt/pull/45725 fixed problems with older
Python 2 versions interacting with the struct module, since
unicode_literals turned the format strings to unicode and older Python 2
releases can't accept a unicode format string. Since Python 2 and 3 both
support using bytestrings for the format strings, this commit makes sure
we are using bytestrings everywhere else we are using struct.pack/unpack.
