Update idna to 2.8

[pyup-bot]

This PR updates idna from 2.5 to 2.8.

Changelog

2.8

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

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

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.

2.6

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

- Allows generation of IDNA and UTS 46 table data for different
versions of Unicode, by deriving properties directly from
Unicode data.
- Ability to generate RFC 5892/IANA-style table data
- Diagnostic output of IDNA-related Unicode properties and
derived calculations for a given codepoint
- Support for idna.__version__ to report version
- Support for idna.idnadata.__version__ and
idna.uts46data.__version__ to report Unicode version of
underlying IDNA and UTS 46 data respectively.

Links

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

[jluebbe]

Crossbar currently pins to idna 2.5.