build(deps-dev): bump responses from 0.17.0 to 0.21.0 in /backend

Bumps [responses](https://github.com/getsentry/responses) from 0.17.0 to 0.21.0.
- [Release notes](https://github.com/getsentry/responses/releases)
- [Changelog](https://github.com/getsentry/responses/blob/master/CHANGES)
- [Commits](getsentry/responses@0.17.0...0.21.0)

---
updated-dependencies:
- dependency-name: responses
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

backend/requirements-dev.txt

@@ -2,4 +2,4 @@ fakeredis==1.7.0
 pre-commit==2.17.0
 pytest==6.2.5
 pytest-responses==0.5.0
-responses==0.17.0
+responses==0.21.0