build(deps-dev): Bump responses from 0.10.14 to 0.10.15 in /backend (#548)

Bumps [responses](https://github.com/getsentry/responses) from 0.10.14 to 0.10.15.
- [Release notes](https://github.com/getsentry/responses/releases)
- [Changelog](https://github.com/getsentry/responses/blob/master/CHANGES)
- [Commits](getsentry/responses@0.10.14...0.10.15)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

backend/requirements-dev.txt

@@ -2,4 +2,4 @@ fakeredis==1.4.1
 pre-commit==2.5.1
 pytest==5.4.3
 pytest-responses==0.4.0
-responses==0.10.14
+responses==0.10.15