Bump idna from 2.10 to 3.1

Bumps [idna](https://github.com/kjd/idna) from 2.10 to 3.1.
- [Release notes](https://github.com/kjd/idna/releases)
- [Changelog](https://github.com/kjd/idna/blob/master/HISTORY.rst)
- [Commits](kjd/idna@v2.10...v3.1)

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

requirements.txt

@@ -2,7 +2,7 @@ certifi==2020.12.05
 babel==2.9.0
 flask-babel==2.0.0
 flask==1.1.2
-idna==2.10
+idna==3.1
 jinja2==2.11.3
 lxml==4.6.2
 pygments==2.8.0