Update tox to 2.9.1

[pyup-bot]

There's a new version of tox available.
You are currently using 2.8.2. I have updated it to 2.9.1

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

Changelog

2.9.1

---

Misc
^^^^

• integrated new release process and fixed changelog rendering for pypi.org -
  by obestwalter <https://github.com/obestwalter>_.

2.9

2.9.0

---

Features
^^^^^^^^

• tox --version now shows information about all registered plugins - by
  obestwalter <https://github.com/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 ferdonline <https://github.com/ferdonline>_
  (571 <https://github.com/tox-dev/tox/issues/571>_)

Misc
^^^^

• 635 <https://github.com/tox-dev/tox/issues/635>_ inherit from correct exception -
  by obestwalter <https://github.com/obestwalter>_
  (635 <https://github.com/tox-dev/tox/issues/635>_).
• spelling and escape sequence fixes - by scoop <https://github.com/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 obestwalter <https://github.com/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>_)

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

Happy merging! 🤖