Update twine to 2.0.0

[pyup-bot]

This PR updates twine from 1.15.0 to 2.0.0.

Changelog

2.0.0

* :feature:`437`: Twine now requires Python 3.6 or later. Use pip
9 or pin to "twine<2" to install twine on older Python versions.
* :bug:`491` Require requests 2.20 or later to avoid reported security
vulnerabilities in earlier releases.

Links

• PyPI: https://pypi.org/project/twine
• Changelog: https://pyup.io/changelogs/twine/
• Docs: https://twine.readthedocs.io/

[pyup-bot]

Closing this in favor of #86