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* Major restructuring of code to work with poetry rather than setuptools.
* Update and simplification of code to work with Sphinx >= 3.4.2. Prior
versions relied on some private behaviour which is deprecated. The new
version can now use the public API.
* Resolves issue 1 by inserting a small javascript snippet that changes
``class="py interface"`` to ``class="py interface class"`` so that themes
which don't provide support for styling interface (most of them) will
fallback to using the same format as a class.