Fix small numpydoc bug so docs build out of the box.

Already asked on numpy list if the fix is OK to upstream.

docs/sphinxext/numpydoc.py

@@ -68,7 +68,8 b' def mangle_docstrings(app, what, name, obj, options, lines,'
 def mangle_signature(app, what, name, obj, options, sig, retann):
     # Do not try to inspect classes that don't define `__init__`
     if (inspect.isclass(obj) and
-        'initializes x; see ' in pydoc.getdoc(obj.__init__)):
+        (not hasattr(obj, '__init__') or
+        'initializes x; see ' in pydoc.getdoc(obj.__init__))):
         return '', ''
 
     if not (callable(obj) or hasattr(obj, '__argspec_is_invalid_')): return