Update idna to 2.8

[pyup-bot]

This PR updates idna from 2.7 to 2.8.

Changelog

2.8

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

- Update to Unicode 11.0.0.
- Provide more specific exceptions for some malformed labels.

Links

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

[eviljeff]

https://github.com/requests/requests/blob/v2.20.1/setup.py#L46 has a requirement for IDNA<2.8 psf/requests#4890 asks for the requirement to be bumped so this is blocked on a new release of requests. (we could pin if it's going to be a while)

[EnTeQuAk]

Should be fixed via #10190