Tab completion broken for rez deployments installed with Python-3.6

[zachlewis]

If one installs rez with the system python3 interpreter, instead of the inspected tab completion behavior, one gets a faceful of this:

Traceback (most recent call last):
  File "/opt/rez/bin/rez/_rez-complete", line 8, in <module>
    sys.exit(run_rez_complete())
  File "/opt/rez/lib/python3.6/site-packages/rez/cli/_entry_points.py", line 80, in run_rez_complete
    return run("complete")
  File "/opt/rez/lib/python3.6/site-packages/rez/cli/_main.py", line 170, in run
    returncode = run_cmd()
  File "/opt/rez/lib/python3.6/site-packages/rez/cli/_main.py", line 162, in run_cmd
    return func(opts, opts.parser, extra_arg_groups)
  File "/opt/rez/lib/python3.6/site-packages/rez/cli/complete.py", line 97, in command
    comp_point=comp_point)
  File "/opt/rez/lib/python3.6/site-packages/rez/cli/_complete_util.py", line 16, in __init__
    comp_point = len(comp_line[:comp_point].decode(sys_encoding))
AttributeError: 'str' object has no attribute 'decode'