Add responses as a test dependency

Author: yuokada

setup.py

@@ -41,6 +41,7 @@
     # httpretty >= 1.1 duplicates requests in `httpretty.latest_requests`
     # https://github.com/gabrielfalcao/HTTPretty/issues/425
     "httpretty < 1.1",
+    "responses>=0.24.1,<1",
     "pytest",
     "pytest-runner",
     "pre-commit",