Fix leak of file descriptor with `_io.FileIO('dir/')`

Merged Bitbucket Importer requested to merge bitbucket/merged-pr-647 into branch/py3.6

Created originally on Bitbucket by asottile (Anthony Sottile)

Was already merged in Bitbucket before import, marked as merged by the import user

Fix leak of file descriptor with _io.FileIO('dir/')

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