Bump idna from 3.7 to 3.15 in /__tests__/data

Bumps [idna](https://github.com/kjd/idna) from 3.7 to 3.15.
- [Release notes](https://github.com/kjd/idna/releases)
- [Changelog](https://github.com/kjd/idna/blob/master/HISTORY.md)
- [Commits](https://github.com/kjd/idna/compare/v3.7...v3.15)

---
updated-dependencies:
- dependency-name: idna
  dependency-version: '3.15'
  dependency-type: direct:production
...

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

__tests__/data/requirements-linux.txt

@@ -1,7 +1,7 @@
 certifi==2020.6.20
 chardet==3.0.4
 docutils==0.16
-idna==3.7
+idna==3.15
 Kivy==2.0.0rc3
 Kivy-Garden==0.1.4
 packaging==20.7

__tests__/data/requirements.txt

@@ -8,7 +8,7 @@ docutils==0.16
 
 future==0.18.2; python_version >= '2.6' and python_version not in '3.0, 3.1, 3.2'
 
-idna==3.7
+idna==3.15
 
 itsdangerous==1.1.0