AttributeError from contract errback

[StasDeep]

When running a contract with a URL that returns non-200 response, I get the following:

2018-08-09 14:40:23 [scrapy.core.scraper] ERROR: Spider error processing <GET https://www.bureauxlocaux.com/annonce/a-louer-bureaux-a-louer-a-nantes--1289-358662> (referer: None)
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/twisted/internet/defer.py", line 653, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/usr/local/lib/python3.6/site-packages/scrapy/contracts/__init__.py", line 89, in eb_wrapper
    results.addError(case, exc_info)
  File "/usr/local/lib/python3.6/unittest/runner.py", line 67, in addError
    super(TextTestResult, self).addError(test, err)
  File "/usr/local/lib/python3.6/unittest/result.py", line 17, in inner
    return method(self, *args, **kw)
  File "/usr/local/lib/python3.6/unittest/result.py", line 115, in addError
    self.errors.append((test, self._exc_info_to_string(err, test)))
  File "/usr/local/lib/python3.6/unittest/result.py", line 186, in _exc_info_to_string
    exctype, value, tb, limit=length, capture_locals=self.tb_locals)
  File "/usr/local/lib/python3.6/traceback.py", line 470, in __init__
    exc_value.__cause__.__traceback__,
AttributeError: 'getset_descriptor' object has no attribute '__traceback__'

Here is how exc_info looks like:

(HttpError('Ignoring non-200 response',), <class 'scrapy.spidermiddlewares.httperror.HttpError'>, <traceback object at 0x7f4bdca1d948>)

[StasDeep]

I've found the bug. It's inside scrapy/contracts/__init__.py.

Here's how exc_info is initialized:

exc_info = failure.value, failure.type, failure.getTracebackObject()

So we get (value, type, traceback) tuple. But it must be (type, value, traceback).

I'll create a PR with a fix soon.