Bump cryptography from 3.1 to 3.1.1 in /backend

Bumps [cryptography](https://github.com/pyca/cryptography) from 3.1 to 3.1.1.
- [Release notes](https://github.com/pyca/cryptography/releases)
- [Changelog](https://github.com/pyca/cryptography/blob/master/CHANGELOG.rst)
- [Commits](pyca/cryptography@3.1...3.1.1)

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

Author: dependabot[bot]

backend/requirements.txt

@@ -16,7 +16,7 @@ celery==4.4.7
 redis==3.5.3
 
 itsdangerous==1.1.0
-cryptography==3.1
+cryptography==3.1.1
 
 flask_jwt_extended==3.24.1