Bump idna from 2.6 to 2.8

[dependabot-preview]

Bumps idna from 2.6 to 2.8.

Changelog

Sourced from idna's changelog.

2.8 (2018-12-04)

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

2.7 (2018-06-10)

• 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.

Commits

• 1cdf175 Release v2.8
• 27fe03f Don't mask more specific exception (fixes #65)
• 0f50bdc Preliminary Unicode 11.0.0 data
• d075717 Add trove classifier for general Python 2 support (#57)
• 2618ad1 Stop testing EOL Python 2.6 and 3.3 (#62)
• 5d76cb6 Release v2.7
• f8389a8 Release v2.7
• 28dc508 Fix testing regressions caused by Unicode 10.0 data and <10.0 data in base Py...
• 1b377da Unwind bad merge to 2.7-dev HEAD
• c225344 Merge branch 'master' into 2.7-dev
• Additional commits viewable in compare view

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[dependabot-preview]

Looks like idna is up-to-date now, so this is no longer needed.