Update tox to 2.9.0

[pyup-bot]

There's a new version of tox available.
You are currently using 2.8.0. I have updated it to 2.9.0

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

Changelog

2.9.0

==================

Features

• tox --version now shows information about all registered plugins - by
  :user:obestwalter (544 <https://github.com/tox-dev/tox/issues/544>_).

Bugfixes

• skip_install overrides usedevelop (usedevelop is an option to
  choose the installation type if the package is installed and skip_install
  determines if it should be installed at all) - by :user:ferdonline (571 <https://github.com/tox-dev/tox/issues/571>_).

Misc

• :pull:635 inherit from correct exception - by :user:obestwalter
  (635 <https://github.com/tox-dev/tox/issues/635>_).
• spelling and escape sequence fixes - by :user:scoop
  (637 <https://github.com/tox-dev/tox/issues/637>_ and
  638 <https://github.com/tox-dev/tox/issues/638>_).
• add a badge to show build status of documentation on readthedocs.io -
  by :user:obestwalter.

Improved Documentation

• add towncrier <https://github.com/hawkowl/towncrier>_ to allow adding
  changelog entries with the pull requests without generating merge conflicts;
  with this release notes are now grouped into four distinct collections:
  Features, Bugfixes, Improved Documentation and Deprecations and Removals. (614 <https://github.com/tox-dev/tox/issues/614>_)

2.8.2

==================

• 466 <https://github.com/tox-dev/tox/issues/466>_: stop env var leakage if popen failed with resultjson or redirect

2.8.1

==================

• :pull:599: fix problems with implementation of :issue:515.
  Substitutions from other sections were not made anymore if they were not in envlist.
  Thanks to Clark Boylan (:user:cboylan) for helping to get this fixed (:pull:597).

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 61.252% when pulling b2a3f21 on pyup-update-tox-2.8.0-to-2.9.0 into 9a925d5 on master.

[pyup-bot]

Closing this in favor of #14