Bump idna from 3.4 to 3.8

Bumps [idna](https://github.com/kjd/idna) from 3.4 to 3.8.
- [Release notes](https://github.com/kjd/idna/releases)
- [Changelog](https://github.com/kjd/idna/blob/master/HISTORY.rst)
- [Commits](kjd/idna@v3.4...v3.8)

---
updated-dependencies:
- dependency-name: idna
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

docs/requirements.txt

@@ -24,7 +24,7 @@ et-xmlfile==1.1.0
     # via openpyxl
 fonttools==4.51.0
     # via matplotlib
-idna==3.4
+idna==3.8
     # via requests
 imagesize==1.4.1
     # via sphinx