Test failures with Python 3.5

[pylint-bot]

Originally reported by: Brian Lane (BitBucket: bcl, GitHub: @bcl?)

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I ran into a couple of test failures:

LoadModuleFromPathTest was failing on the 2nd pass because sys.just_once had been deleted.

In AstroidManagerTest there may be other hooks already present, so deleting 0 isn't correct. Removing -1 would likely work, but we may as well just look up the index and use that.

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• Bitbucket: https://bitbucket.org/logilab/astroid/issue/274

[pylint-bot]

Original comment by Claudiu Popa (BitBucket: PCManticore, GitHub: @PCManticore?):

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Weird, nothing fails for me or in the CI. Could you send this as a PR? Thanks!