Update twine to 3.0.0

[pyup-bot]

This PR updates twine from 2.0.0 to 3.0.0.

Changelog

3.0.0

* :feature:`336`: When a client certificate is indicated, all password
processing is disabled.
* :feature:`524`: Twine now unconditionally requires the keyring library
and no longer supports uninstalling ``keyring`` as a means to disable
that functionality. Instead, use ``keyring --disable`` keyring functionality
if necessary.
* :feature:`518` Add Python 3.8 to classifiers.
* :bug:`332` More robust handling of server response in ``--skip-existing``

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 #106