Implement bpo-27535: Fix memory leak with warnings ignore

Merged Yannick Jadoul requested to merge topic/py3.7/py3.7-bpo-27535 into branch/py3.7

This should fix the test.test_warnings.CFilterTests failing test in test_warnings.

See &

As far as I know, and cannot/do not have to be implemented, since PyPy doesn't use an equivalent of _Py_IDENTIFIER?

Edited by Yannick Jadoul

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