erp5: add mock egg

https://pypi.org/project/mock/ which is a backport of
https://docs.python.org/3/library/unittest.mock.html

stack/erp5/buildout.cfg

@@ -473,6 +473,7 @@ eggs = ${neoppod:eggs}
   Jinja2
   jsonschema
   mechanize
+  mock
   objgraph
   paramiko
   ply
@@ -801,3 +802,9 @@ unidiff = 0.5.5
 # deepdiff = 3.3.0
 jsonpickle = 0.9.6
 decorator = 4.3.0
+
+mock = 3.0.4
+
+# Required by:
+# mock = 3.0.4
+funcsigs = 1.0.2