make _Miss iterable

[maxnbk]

No description provided.

[nerdvegas]

Closing as doesn't seem necessary (did old code require it?)

[maxnbk]

Reopening because I found the stack trace that was relevant for this.

Traceback (most recent call last):
  File "/rez/bin/rez/rez-env", line 8, in <module>
    sys.exit(run_rez_env())
  File "/rez/lib/python3.7/site-packages/rez/cli/_entry_points.py", line 152, in run_rez_env
    return run("env")
  File "/rez/lib/python3.7/site-packages/rez/cli/_main.py", line 152, in run
    returncode = run_cmd()
  File "/rez/lib/python3.7/site-packages/rez/cli/_main.py", line 144, in run_cmd
    return opts.func(opts, opts.parser, extra_arg_groups)
  File "/rez/lib/python3.7/site-packages/rez/cli/env.py", line 206, in command
    print_stats=opts.stats)
  File "/rez/lib/python3.7/site-packages/rez/resolved_context.py", line 282, in __init__
    resolver.solve()
  File "/rez/lib/python3.7/site-packages/rez/utils/memcached.py", line 249, in wrapper
    return func(*nargs, **kwargs)
  File "/rez/lib/python3.7/site-packages/rez/resolver.py", line 111, in solve
    solver_dict = self._get_cached_solve()
  File "/rez/lib/python3.7/site-packages/rez/resolver.py", line 298, in _get_cached_solve
    if _packages_changed(key, data) or _releases_since_solve(key, data):
  File "/rez/lib/python3.7/site-packages/rez/resolver.py", line 227, in _packages_changed
    solver_dict, _, variant_states_dict = data
TypeError: cannot unpack non-iterable _Miss object

[nerdvegas]

Closing in favor of #755