[pypy-issue] Issue #2760: getattr lookup difference in numpy/cpyext (pypy/pypy)

[pv]

New issue 2760: getattr lookup difference in numpy/cpyext
https://bitbucket.org/pypy/pypy/issues/2760/getattr-lookup-difference-in-numpy-cpyext

pv:

Consider the following code:
```
import numpy as np

class Passthrough(object):
    def __init__(self, data):
        self.data = data
    def __getattr__(self, name):
        print("GETATTR:", name)
        return getattr(self.data, name)

x = Passthrough(np.array([1]))
print("GOT:", np.array(x))
```
On CPython this prints
```
GETATTR: __array_struct__
GOT: [1]
```
On pypy3-5.10.1 and pypy2-5.10.0 it prints
```
GOT: <__main__.Passthrough object at 0x0000000001776aa0>
```
The relevant Numpy code where it tries to look up the attribute is here:

[PyArray_LookupSpecial_OnInstance](https://github.com/numpy/numpy/blob/1ee1a4ac7f79e8c707c416f81f049fd3ac94f1e6/numpy/core/src/private/get_attr_string.h#L119), [PyArray_FromStructInterface](https://github.com/numpy/numpy/blob/1ee1a4ac7f79e8c707c416f81f049fd3ac94f1e6/numpy/core/src/multiarray/ctors.c#L2082), [PyArray_GetArrayParamsFromObject](https://github.com/numpy/numpy/blob/1ee1a4ac7f79e8c707c416f81f049fd3ac94f1e6/numpy/core/src/multiarray/ctors.c#L1500)

Looks mightily suspicious, but is it a cpyext or Numpy issue?


(In other news, scipy test suite has 6 remaining failures (out of 12000+ tests) on pypy3, two of them due to the above.)