build(deps-dev): Bump pre-commit from 1.18.3 to 1.20.0 in /backend

Bumps [pre-commit](https://github.com/pre-commit/pre-commit) from 1.18.3 to 1.20.0.
- [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@v1.18.3...v1.20.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

backend/requirements-dev.txt

@@ -1,5 +1,5 @@
 fakeredis==1.0.5
-pre-commit==1.18.3
+pre-commit==1.20.0
 pytest==5.2.1
 pytest-responses==0.4.0
 responses==0.10.6