Update changelog to match tags (semver style)
-20.0.0
-------
+v20.0.0
+-------
* Extracted ``keyring.testing`` package to contain supporting
functionality for plugin backends. ``keyring.tests`` has been
removed from the package.
-19.3.0
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+v19.3.0
+-------
* Switch to `importlib.metadata
<https://docs.python.org/3/library/importlib.metadata.html>`_
