Update virtualenv to 20.0.0

[pyup-bot]

This PR updates virtualenv from 16.0.0 to 20.0.0.

Changelog

20.0.0b2

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- Create the first iteration of the new documentation - by :user:`gaborbernat`. (`1465 <https://github.com/pypa/virtualenv/issues/1465>`_)
- Project readme is now of type MarkDown instead of reStructuredText - by :user:`gaborbernat`. (`1531 <https://github.com/pypa/virtualenv/issues/1531>`_)

20.0.0b1

----------------------

* First public release of the rewrite. Everything is brand new and just added.

.. warning::

The current virtualenv is the second iteration of implementation. From version ``0.8`` all the way to ``16.7.9``
we numbered the first iteration. Version ``20.0.0b1`` is a complete rewrite of the package, and as such this release
history starts from there.

Links

• PyPI: https://pypi.org/project/virtualenv
• Changelog: https://pyup.io/changelogs/virtualenv/
• Homepage: https://virtualenv.pypa.io/

[coveralls]

Coverage remained the same at ?% when pulling 58a0131 on pyup-update-virtualenv-16.0.0-to-20.0.0 into 571d360 on master.

[pyup-bot]

Closing this in favor of #994