Bump idna from 3.3 to 3.7 in /docs (#670)

Bumps [idna](https://github.com/kjd/idna) from 3.3 to 3.7.
- [Release notes](https://github.com/kjd/idna/releases)
- [Changelog](https://github.com/kjd/idna/blob/master/HISTORY.rst)
- [Commits](kjd/idna@v3.3...v3.7)

---
updated-dependencies:
- dependency-name: idna
  dependency-type: indirect
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

docs/requirements.txt

@@ -18,7 +18,7 @@ docutils==0.17.1
     # via
     #   sphinx
     #   sphinx-rtd-theme
-idna==3.3
+idna==3.7
     # via requests
 imagesize==1.3.0
     # via sphinx