Add missing `ensured-targets`

[jtpio]

Fix https://github.com/jupyterlite/xeus-python-kernel/issues/177.

Which could be related to https://github.com/jupyterlite/xeus-python-kernel/pull/168

• [x] Add missing ensured-targets
• [x] Align build workflow with the extension template
• [x] Fix wheel packaging

[github-actions[bot]]

:point_left: Try it on ReadTheDocs

[jtpio]

OK this should fix the issue. The size of the artifacts (sdist + wheel) looks more correct now:

Will be making a new alpha release right after to test it.