erp5: add responses egg

https://github.com/getsentry/responses A utility for mocking out the
Python Requests library.

stack/erp5/buildout.cfg

@@ -483,6 +483,7 @@ eggs = ${neoppod:eggs}
   python-memcached
   pytz
   requests
+  responses
   threadframe
   timerserver
   urlnorm
@@ -808,3 +809,9 @@ mock = 3.0.4
 # Required by:
 # mock = 3.0.4
 funcsigs = 1.0.2
+
+responses = 0.10.6
+
+# Required by:
+# responses = 0.10.6
+cookies = 2.2.1