setup: declare test dependences with extras_require

Only setup.py test understands tests_require

setup.py

@@ -5,6 +5,13 @@ name = "slapos.rebootstrap"
 long_description = open("README.rst").read() + '\n\n'
 long_description += open("CHANGELOG.rst").read()
 
+extras_require = {
+    'test': [
+        'zope.testing',
+        'manuel',
+    ]
+}
+
 setup(
   name=name,
   version=version,
@@ -38,9 +45,7 @@ setup(
     'setuptools',
     'zc.buildout >=2.7.1+slapos001, <2.7.2',
     ],
-  tests_require=[
-      'zope.testing',
-      'manuel'
-    ],
+  extras_require=extras_require,
+  tests_require=extras_require['test'],
   test_suite='%s.tests.test_suite' % name,
 )