Bump pre-commit from 2.1.0 to 2.4.0 in /backend (#155)

Bumps [pre-commit](https://github.com/pre-commit/pre-commit) from 2.1.0 to 2.4.0.
- [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@v2.1.0...v2.4.0)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

backend/requirements.txt

@@ -22,7 +22,7 @@ flask_jwt_extended==3.24.1
 
 flake8==3.7.9
 black==19.10b0
-pre-commit==2.1.0
+pre-commit==2.4.0
 
 names==0.3.0
 lorem==0.1.1