Update tox to 2.8.0

[pyup-bot]

There's a new version of tox available.
You are currently using 2.7.0. I have updated it to 2.8.0

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

Changelog

2.8rc2

---

• 276: Remove easy_install from docs (TL;DR: use pip).
  Thanks Martin Andrysík (sifuraz).
• 301: Expand nested substitutions in tox.ini
  Thanks vlaci.
  Thanks to Eli Collins (eli-collins) for creating a reproducer.
• 315: add --help and --version to helptox-quickstart
  Thanks vlaci.
• 326: Fix OSError 'Not a directory' when creating env on Jython 2.7.0.
  Thanks Nick Douma (LordGaav).
• 429: Forward MSYSTEM by default on Windows
  Thanks Marius Gedminas (mgedmin) for reporting this.
• 449: add multi platform example to the docs.
  Thanks Aleks Bunin (sashkab) and rndr.
• 474: Start using setuptools_scm for tag based versioning.
• 484: Renamed py.test to pytest throughout the project
  Thanks Slam (3lnc).
• 504: With -a: do not show additional environments header if there are none
  Thanks rndr.
• 515: Don't require environment variables in test environments where they
  are not used.
  Thanks André Caron (AndreLouisCaron).
• 517: Forward NUMBER_OF_PROCESSORS by default on Windows to fix
  multiprocessor.cpu_count().
  Thanks André Caron (AndreLouisCaron).
• 518: Forward USERPROFILE by default on Windows.
  Thanks André Caron (AndreLouisCaron).
• p528: Fix some of the warnings displayed by pytest 3.1.0.
  Thanks Bruno Oliveira (nicoddemus).
• p547: Add regression test for 137
  Thanks Martin Andrysík (sifuraz).
• p553: Add an XFAIL test to reproduce upstream bug 203
  Thanks Bartolomé Sánchez Salado (bartsanchez).
• p556: Report more meaningful errors on why virtualenv creation failed
  Thanks vlaci.
  Also thanks to Igor Sadchenko (igor-sadchenko) for pointing out a
  problem with that PR before it hit the masses :)
• 575: Add announcement doc to end all announcement docs
  (using only CHANGELOG and Github issues since 2.5 already)
• p580: Do not ignore Sphinx warnings anymore
  Thanks Gábor Bernát (gaborbernat).
• 585: Expand documentation to explain pass through of flags from deps to pip
  (e.g. -rrequirements.txt, -cconstraints.txt)
  Thanks Alexander Loechel (loechel).
• 588: Run pytest wit xfail_strict and adapt affected tests.

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

Happy merging! 🤖

[pyup-bot]

Closing this in favor of #23