Bump pre-commit from 2.5.1 to 2.7.1 in /backend

Bumps [pre-commit](https://github.com/pre-commit/pre-commit) from 2.5.1 to 2.7.1.
- [Release notes](https://github.com/pre-commit/pre-commit/releases)
- [Changelog](https://github.com/pre-commit/pre-commit/blob/master/CHANGELOG.md)
- [Commits](pre-commit/pre-commit@v2.5.1...v2.7.1)

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

Author: dependabot[bot]

backend/requirements.txt

@@ -22,7 +22,7 @@ flask_jwt_extended==3.24.1
 
 flake8==3.8.3
 black==19.10b0
-pre-commit==2.5.1
+pre-commit==2.7.1
 
 names==0.3.0
 lorem==0.1.1