bpo-31284: IDLE: Fix WindowList warning message when running tests

[csabella]

https://bugs.python.org/issue31284

[terryjreedy]

I think this is more likely a cover-up than a real fix.

The subsequent experiments discussed on the tracker convince me more of this. Masking the warning would be a last resort for getting a test to close cleanly.

[bedevere-bot]

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Once you have made the requested changes, please leave a comment on this pull request containing the phrase I didn't expect the Spanish Inquisition!. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[terryjreedy]

The problem in bpo-30617 is fixed by calling close.