Merge branch 'tox_remove_py25' into 'master'

Tox remove py25

Now maybe py25 is important to support and maybe you have a CI process that installs some older version of tox so that you can test Python 2.5, in which case you can close this, though if that's the case then maybe we should make a note of that fact and what version of tox to use, etc.

See merge request !22