Do not restrict version of ExtensionClass.

Plone 5 is using ExtensionClass 4.1.2, in combination with Acquisition 4.2.2.
That is running fine.
And bin/test in DocumentTemplate runs fine when I use those version pins.

setup.py

@@ -35,7 +35,7 @@
       install_requires=[
         'AccessControl',
         'Acquisition',
-        'ExtensionClass<4.0.dev',
+        'ExtensionClass',
         'RestrictedPython',
         'zExceptions',
         'zope.sequencesort',