Bump idna from 2.10 to 3.7 in /docs

Bumps [idna](https://github.com/kjd/idna) from 2.10 to 3.7.
- [Release notes](https://github.com/kjd/idna/releases)
- [Changelog](https://github.com/kjd/idna/blob/master/HISTORY.rst)
- [Commits](kjd/idna@v2.10...v3.7)

---
updated-dependencies:
- dependency-name: idna
  dependency-type: direct:production
...

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

docs/requirements.txt

@@ -13,7 +13,7 @@ decorator==4.4.2
 defusedxml==0.6.0
 docutils==0.16
 entrypoints==0.3
-idna==2.10
+idna==3.7
 imagesize==1.2.0
 importlib-metadata==2.0.0
 ipython-genutils==0.2.0