Do not asssume Item.obj in 'skipping' plugin

[vidartf]

See #2231 for discussion.

Not entirely sure how to best wire up a regression test for this, but the following code should do the trick once collected:

class CustomItem(pytest.Item):
    def setup(self):
        marker = pytest.mark.xfail(True)
        self.add_marker(marker)

    def runtest(self):
        assert False

[nicoddemus]

Hey @vidartf thanks for following up with this! 👍

Could you use the code you suggest along with the other files necessary (conftest.py file with custom collection hook, etc) which proves the patch work? If so, we can add an integration test in test_skipping.

[coveralls]

Coverage increased (+0.0008%) to 92.833% when pulling 1a88a91 on vidartf:patch-1 into da5a3db on pytest-dev:master.

[coveralls]

Coverage increased (+0.0008%) to 92.833% when pulling 832c89d on vidartf:patch-1 into da5a3db on pytest-dev:master.

[vidartf]

@nicoddemus I added a test, and after an initial mistake, it seems now to pass. Is this sufficient?

Note: The mistaken test strangely passed on some versions of python. Is this a problem with CI? You will have to check CI logs to see it, as I rebased to keep git history clean.

[nicoddemus]

Very nice, thank you!