Update git submodules

* Update placement from branch 'master'
  - Merge "Use tox 3.1.1 fixes"
  - Use tox 3.1.1 fixes
    
    By 3.1.1, tox had fixed:
    - [1] which mishandled the basepython directive.
    - ignoring PYTHONDONTWRITEBYTECODE [2]
    
    Require this version and remove the workarounds.
    
    See ML thread [3] for some details
    
    [1] tox-dev/tox#477
    [2] tox-dev/tox@336f4f6
    [3] http://lists.openstack.org/pipermail/openstack-dev/2018-July/thread.html#132075
    
    Change-Id: I87cbf4b26bdd40a8b7a699966e1ace838a2491de