Bump jinja2 from 2.11.1 to 2.11.3 in /backend

Bumps [jinja2](https://github.com/pallets/jinja) from 2.11.1 to 2.11.3. - [Release notes](https://github.com/pallets/jinja/releases) - [Changelog](https://github.com/pallets/jinja/blob/master/CHANGES.rst) - [Commits](pallets/jinja@2.11.1...2.11.3) Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

backend/requirements.txt

@@ -6,7 +6,7 @@ Django==3.0.7
 djangorestframework==3.11.0
 idna==2.9
 itypes==1.1.0
-Jinja2==2.11.1
+Jinja2==2.11.3
 MarkupSafe==1.1.1
 pytz==2019.3
 requests==2.23.0