[issue21729] Use `with` statement in dbm.dumb

[Raymond Hettinger]

Raymond Hettinger added the comment:

Serhiy, after reviewing this, consider backporting it.

The original code doesn't have a try/finally around the close() call and that could be considered a bug.

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nosy: +rhettinger

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21729>
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