Handle struct issues without depending on future in Py3
Fix to re-remove future dependency for Py3
Remove Py3-specific string handling that should not be necessary
Proper struct.pack() workaround: use b'' for format string

With unicode_literals, '' on Py2 is not a native string, but b'' is, and can be
used in struct.pack(). b'' on Py3 is a native bytes, but struct.pack() is happy
to use it.
have RVLS message make more than 1 lease set