Error loading into mypy

When I use mypy with these stubs (python 3.6, MacOS) I get this (cryptic) message:
```
stubs/numpy/__init__.pyi:48: error: Class numpy.flatiter has abstract attributes "__next__"
stubs/numpy/__init__.pyi:48: note: If it is meant to be abstract, add 'abc.ABCMeta' as an explicit metaclass
```
I'm afraid I don't know what this means; I suspect it has to do with the use of type variables, but...