Update idna to 2.9

[pyup-bot]

This PR updates idna from 2.8 to 2.9.

Changelog

2.9

++++++++++++++++

- Update to Unicode 12.1.0.
- Prohibit A-labels ending with a hyphen (Thanks, Julien Bernard!)
- Future-proofing: Test on Python 3.7 and 3.8, don't immediately
fail should Python 4 come along.
- Made BSD 3-clause license clearer

Links

• PyPI: https://pypi.org/project/idna
• Changelog: https://pyup.io/changelogs/idna/
• Repo: https://github.com/kjd/idna

[pyup-bot]

Closing this in favor of #252