[issue29672] `catch_warnings` context manager should reset warning registry to previous state upon exiting, to prevent warnings from being reprinted

[Gerrit Holl]

Changes by Gerrit Holl <gerrit.holl at gmail.com>:


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title: `catch_warnings` context manager causes warnings to be reprinted -> `catch_warnings` context manager should reset warning registry to previous state upon exiting, to prevent warnings from being reprinted

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue29672>
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