Bump idna from 2.8 to 2.9

Bumps [idna](https://github.com/kjd/idna) from 2.8 to 2.9.
- [Release notes](https://github.com/kjd/idna/releases)
- [Changelog](https://github.com/kjd/idna/blob/master/HISTORY.rst)
- [Commits](kjd/idna@v2.8...v2.9)

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

requirements.txt

@@ -9,7 +9,7 @@ cornice-swagger==0.7.0
 dockerflow==2018.4.0
 elasticsearch==6.3.1
 hupper==1.6.1
-idna==2.8
+idna==2.9
 iso8601==0.1.12
 jsonpatch==1.23
 jsonpointer==2.0