[issue23839] Clear caches after every test

[Martin Panter]

Martin Panter added the comment:

When I run the tests with -Werror (or any other -W option), I now get

2 tests altered the execution environment:
    test___all__ test_warnings

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nosy: +martin.panter

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue23839>
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