build(deps): bump idna from 2.9 to 3.7

Bumps [idna](https://github.com/kjd/idna) from 2.9 to 3.7.
- [Release notes](https://github.com/kjd/idna/releases)
- [Changelog](https://github.com/kjd/idna/blob/master/HISTORY.rst)
- [Commits](kjd/idna@v2.9...v3.7)

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

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

requirements.txt

@@ -68,7 +68,7 @@ govuk-country-register==0.5.0
     # via digitalmarketplace-utils
 govuk-frontend-jinja @ git+https://github.com/Crown-Commercial-Service/govuk-frontend-jinja.git@v0.5.6-alpha
     # via -r requirements.in
-idna==2.9
+idna==3.7
     # via requests
 inflection==0.3.1
     # via digitalmarketplace-content-loader