Add a change note for PR aio-libs#915

CHANGES/837.contrib.rst

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+915.contrib.rst

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+The test framework has been refactored. In the previous state, the circular
+imports reported in :issue:`837`: caused the C-extension tests to be skipped.
+
+Now, there is a set of the ``pytest`` fixtures that is set up in a parametrized
+manner allowing to have a consistent way of accessing mirrored ``multidict``
+implementations across all the tests.
+
+This change also implemented a pair of CLI flags (``--c-extensions`` /
+``--no-c-extensions``) that allow to explicitly request deselecting the tests
+running against the C-extension.
+
+-- by :user:`webknjaz`.