Merge branch 'responses-dev-only' into 'master'

Make responses a dev-only dependency

Closes #485

See merge request !1012

requirements.txt

@@ -19,7 +19,6 @@ python-memcached==1.59
 pytz==2020.1
 PyYAML==5.3.1
 requests==2.24.0
-responses==0.10.7
 sentry-sdk==0.18.0
 tenacity==6.2
 transkribus-client>=0.1.1

tests-requirements.txt

 coverage
 django-nose
 mock~=4.0.0
-responses
+responses==0.12.0
 tripoli