Update tox to 2.6.0

[pyup-bot]

There's a new version of tox available.
You are currently using 2.5.0. I have updated it to 2.6.0

These links might come in handy: PyPI | Changelog | Docs

Changelog

2.6.0

---

• add "alwayscopy" config option to instruct virtualenv to always copy
  files instead of symlinking. Thanks Igor Duarte Cardoso (igordcard).

• pass setenv variables to setup.py during a usedevelop install.
  Thanks Eli Collins (eli-collins).

• replace all references to testrun.org with readthedocs ones.
  Thanks Oliver Bestwalter (obestwalter).

• fix 323 by avoiding virtualenv14 is not used on py32
  (although we don't officially support py32).
  Thanks Jason R. Coombs (jaraco).

• add Python 3.6 to envlist and CI.
  Thanks Andrii Soldatenko (andriisoldatenko).

• fix glob resolution from TOX_TESTENV_PASSENV env variable
  Thanks Allan Feldman (a-feld).

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖