Update tox to 3.1.2 (#56)

This PR updates [tox](https://pypi.org/project/tox) from **3.0.0** to **3.1.2**.



<details>
  <summary>Changelog</summary>
  
  
   ### 3.1.1
   ```
   ------------------

Bugfixes
^^^^^^^^

- PyPi documentation for ``3.1.0`` is broken. Added test to check for this, and
  fix it by :user:`gaborbernat`. (`879
  &lt;https://github.com/tox-dev/tox/issues/879&gt;`_)
   ```
   
  
  
   ### 3.1.0
   ```
   ------------------

Bugfixes
^^^^^^^^

- Add ``ignore_basepython_conflict``, which determines whether conflicting
  ``basepython`` settings for environments containing default factors, such as
  ``py27`` or ``django18-py35``, should be ignored or result in warnings. This
  was a common source of misconfiguration and is rarely, if ever, desirable from
  a user perspective - by :user:`stephenfin` (`477 &lt;https://github.com/tox-dev/tox/issues/477&gt;`_)
- Fix bug with incorrectly defactorized dependencies (deps passed to pip were not de-factorized) - by :user:`bartsanchez` (`706 &lt;https://github.com/tox-dev/tox/issues/706&gt;`_)


Features
^^^^^^^^

- Add support for multiple PyPy versions using default factors. This allows you
  to use, for example, ``pypy27`` knowing that the correct intepreter will be
  used by default - by :user:`stephenfin` (`19 &lt;https://github.com/tox-dev/tox/issues/19&gt;`_)
- Add support to explicitly invoke interpreter directives for environments with
  long path lengths. In the event that ``tox`` cannot invoke scripts with a
  system-limited shebang (e.x. a Linux host running a Jenkins Pipeline), a user
  can set the environment variable ``TOX_LIMITED_SHEBANG`` to workaround the
  system&#39;s limitation (e.x. ``export TOX_LIMITED_SHEBANG=1``) - by :user:`jdknight` (`794 &lt;https://github.com/tox-dev/tox/issues/794&gt;`_)
- introduce a constants module to be used internally and as experimental API - by :user:`obestwalter` (`798 &lt;https://github.com/tox-dev/tox/issues/798&gt;`_)
- Make ``py2`` and ``py3`` aliases also resolve via ``py`` on windows by :user:`asottile`. This enables the following things:
  ``tox -e py2`` and ``tox -e py3`` work on windows (they already work on posix); and setting ``basepython=python2`` or ``basepython=python3`` now works on windows. (`856 &lt;https://github.com/tox-dev/tox/issues/856&gt;`_)
- Replace the internal version parsing logic from the not well tested `PEP-386 &lt;https://www.python.org/dev/peps/pep-0386/&gt;`_ parser for the more general `PEP-440 &lt;https://www.python.org/dev/peps/pep-0440/&gt;`_. `packaging &gt;= 17.1 &lt;https://pypi.org/project/packaging/&gt;`_ is now an install dependency by :user:`gaborbernat`. (`860 &lt;https://github.com/tox-dev/tox/issues/860&gt;`_)


Documentation
^^^^^^^^^^^^^

- extend the plugin documentation and make lot of small fixes and improvements - by :user:`obestwalter` (`797 &lt;https://github.com/tox-dev/tox/issues/797&gt;`_)
- tidy up tests - remove unused fixtures, update old cinstructs, etc. - by :user:`obestwalter` (`799 &lt;https://github.com/tox-dev/tox/issues/799&gt;`_)
- Various improvements to documentation: open browser once documentation generation is done, show Github/Travis info on documentation page, remove duplicate header for changelog, generate unreleased news as DRAFT on top of changelog, make the changelog page more compact and readable (width up to 1280px) by :user:`gaborbernat` (`859 &lt;https://github.com/tox-dev/tox/issues/859&gt;`_)


Miscellaneous
^^^^^^^^^^^^^

- filter out unwanted files in package - by :user:`obestwalter` (`754 &lt;https://github.com/tox-dev/tox/issues/754&gt;`_)
- make the already existing implicit API explicit - by :user:`obestwalter` (`800 &lt;https://github.com/tox-dev/tox/issues/800&gt;`_)
- improve tox quickstart and corresponding tests - by :user:`obestwalter` (`801 &lt;https://github.com/tox-dev/tox/issues/801&gt;`_)
- tweak codecov settings via .codecov.yml - by :user:`obestwalter` (`802 &lt;https://github.com/tox-dev/tox/issues/802&gt;`_)
   ```
   
  
</details>


 

<details>
  <summary>Links</summary>
  
  - PyPI: https://pypi.org/project/tox
  - Changelog: https://pyup.io/changelogs/tox/
  - Docs: https://tox.readthedocs.org/
</details>

dev-requirements.txt

@@ -3,7 +3,7 @@ invoke==1.1.0
 
 # Testing
 pytest==3.6.3
-tox==3.0.0
+tox==3.1.2
 
 # Syntax checking
 flake8==3.5.0