Pin ExtensionClass < 4.0dev.

Pull in a copy of ExtensionClass.h from 2.13.2 (we don't have 'svn:external'
to kick around any more).

setup.py

@@ -36,7 +36,9 @@ setup(name='Acquisition',
                                            '_Acquisition.c')],
                              include_dirs=['include', 'src']),
                    ],
-      install_requires=['ExtensionClass', 'zope.interface'],
+      install_requires=[
+        'ExtensionClass<4.0.dev',
+        'zope.interface'],
       include_package_data=True,
       zip_safe=False,
       )