Add pure Python wheel to release builds

[nightraise]

This adds a pure Python wheel (eg. bitstruct-8.19.0-py3-none-any.whl) to the output builds for upload to PyPI: https://pypi.org/project/bitstruct/#files.

One useful consequence of this is letting us use packages relying on bitstruct in the browser via Pyodide, which can easily install pure Python wheels.

[jnelson]

I'd like to second at least reviewing this PR. If it can be merged, cantools could be updated to run on device architectures not otherwise supported by this library.