Bump idna from 3.7 to 3.15 in /docs

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](kjd/idna@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>

Author: dependabot[bot]

docs/requirements.txt

@@ -4,7 +4,7 @@ Babel==2.12.1
 cffi==1.15.1
 charset-normalizer==2.0.12
 docutils==0.16
-idna==3.7
+idna==3.15
 imagesize==1.4.1
 importlib-metadata==6.8.0
 Jinja2==3.1.6