[deps] Update responses requirement from ~=0.25.3 to ~=0.25.6

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](getsentry/responses@0.25.3...0.25.6)

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

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

Author: dependabot[bot]

requirements-test.txt

@@ -1,5 +1,5 @@
 nose2[coverage_plugin]>=0.6.5
-responses~=0.25.3
+responses~=0.25.6
 openwisp-utils[qa] @ https://github.com/openwisp/openwisp-utils/tarball/1.2
 parameterized~=0.9.0
 freezegun==1.5.1