Add a dependency on the empty `Products.Sessions` project.

The 3.0 release of the project contains no code. It allows projects
to declare a dependency on it for forward compatibility with Zope 4.

doc/CHANGES.rst

@@ -8,6 +8,8 @@ http://docs.zope.org/zope2/
 2.13.25 (unreleased)
 --------------------
 
+- Add a dependency on the empty `Products.Sessions` project.
+
 - Removed docstrings from some methods to avoid publishing them.  From
   Products.PloneHotfix20160419.  [maurits]
 

setup.py

@@ -56,6 +56,7 @@ setup(name='Zope2',
       'MultiMapping',
       'Persistence',
       'Products.OFSP >= 2.13.2',
+      'Products.Sessions',
       'Products.ZCatalog',
       'Products.ZCTextIndex',
       'Record',

version_ranges.cfg

@@ -17,6 +17,7 @@ Products.MailHost = < 2.14.dev
 Products.MIMETools = < 2.14.dev
 Products.OFSP = < 2.14.dev
 Products.PythonScripts = < 2.14.dev
+Products.Sessions = < 4.0.dev
 Products.StandardCacheManagers = < 2.14.dev
 Products.ZCatalog = < 2.14.dev
 Products.ZCTextIndex = < 2.14.dev