Update idna to 2.7

[pyup-bot]

This PR updates idna from 2.6 to 2.7.

Changelog

2.7

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

- Update to Unicode 10.0.0.
- No longer accepts dot-prefixed domains (e.g. ".example") as valid.
This is to be more conformant with the UTS 46 spec. Users should
strip dot prefixes from domains before processing.

Links

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

[glasserc]

Blocked on psf/requests#4681.

[glasserc]

Rebased onto current master.