Make responses a dev-only dependency

Responses is in the main requirements.txt, but it is only used (and should only be used) in unit tests.

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mentioned in commit 84437bea

mentioned in merge request !1012 (merged)

mentioned in merge request !986 (closed)

mentioned in commit f93fe077

mentioned in commit 84f3209d

mentioned in commit ef50140b

mentioned in commit 61edfef1

closed

mentioned in commit 9ecc2e1b