Install the ipaddress package only on Python 2.7.

It is part of the stdlib in Python 3.

CHANGES.rst

@@ -78,6 +78,10 @@ Bugfixes
   to interact well with plone.testing's patching of the global site manager.
   (`#361 <https://github.com/zopefoundation/Zope/pull/361>`_)
 
+- Install the `ipaddress` package only on Python 2.7 as it is part of the
+  stdlib in Python 3.
+  (`#368 <https://github.com/zopefoundation/Zope/issues/368>`_)
+
 
 4.0b5 (2018-05-18)
 ------------------

setup.py

@@ -75,7 +75,6 @@ def _read_file(filename):
         'RestrictedPython',
         'ZConfig >= 2.9.2',
         'ZODB',
-        'ipaddress',
         'setuptools',
         'six',
         'transaction',
@@ -117,6 +116,11 @@ def _read_file(filename):
         'zope.traversing',
         'zope.viewlet',
     ],
+    extras_require={
+        ':python_version=="2.7"': [
+            'ipaddress',
+        ]
+    },
     include_package_data=True,
     zip_safe=False,
     entry_points={