[deps] Update responses requirement from ~=0.25.8 to ~=0.26.0

Updates the requirements on [responses](https://github.com/getsentry/responses) to permit the latest version.
- [Release notes](https://github.com/getsentry/responses/releases)
- [Changelog](https://github.com/getsentry/responses/blob/master/CHANGES)
- [Commits](https://github.com/getsentry/responses/compare/0.25.8...0.26.0)

---
updated-dependencies:
- dependency-name: responses
  dependency-version: 0.26.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Federico Capoano <f.capoano@openwisp.io>