Comment 1 for bug 1806089

I've reproduced this issue on packstack rocky deployment:
1. clone tempest master branch
2. Install tox python3 tox environment and source it:
  $ tox -epy36
  $ source .tox/py36/bin/activate
3. Create saved_state.json and run cleanup:
  $ tempest cleanup --init-saved-state
  $ tempest cleanup
<the traceback as in the comment above>

The issue seems to be in python3 codecs library which expects to have new data in byte format not in string one like it is in python2.
I've found the same problem described on stackoverflow:
https://stackoverflow.com/questions/45037676/python-3-6-urllib-typeerror-cant-concat-bytes-to-str

So I guess, we just need to change the mode for reading the saved_state.json file.