Update twine to 2.0.0

[pyup-bot]

This PR updates twine from 1.13.0 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.

1.15.0

* :feature:`488` Improved output on ``check`` command:
Prints a message when there are no distributions given to check.
Improved handling of errors in a distribution's markup, avoiding
messages flowing through to the next distribution's errors.

1.14.0

* :feature:`456` Better error handling and gpg2 fallback if gpg not available.
* :bug:`341` Fail more gracefully when encountering bad metadata
* :feature:`459` Show Warehouse URL after uploading a package
* :feature:`310` Now provide a more meaningful error on redirect during upload.

Links

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