Update tox to 2.8.1

[pyup-bot]

There's a new version of tox available.
You are currently using 2.7.0. I have updated it to 2.8.1

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

Changelog

2.8.1

---

• p599: fix problems with implementation of 515.
  Substitutions from other sections were not made anymore if they were not in envlist.
  Thanks to Clark Boylan (cboylan) for helping to get this fixed (p597).

2.8.0

---

• 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! 🤖

[coveralls]

Coverage remained the same at 100.0% when pulling ca3d2d6 on pyup-update-tox-2.7.0-to-2.8.1 into e3ee1b9 on master.

[pyup-bot]

Closing this in favor of #32