Bump idna from 2.6 to 2.8

Bumps [idna](https://github.com/kjd/idna) from 2.6 to 2.8.
- [Release notes](https://github.com/kjd/idna/releases)
- [Changelog](https://github.com/kjd/idna/blob/master/HISTORY.rst)
- [Commits](kjd/idna@v2.6...v2.8)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Author: dependabot-support

requirements.txt

@@ -6,7 +6,7 @@ django-dotenv==1.4.2
 django-webpack-loader==0.6.0
 eventbrite==3.3.3
 gunicorn==19.9.0
-idna==2.6
+idna==2.8
 opbeat==3.5.2
 psycopg2-binary==2.7.7
 pytz==2018.9