Add tests and doc to sdist

[maresb]

This makes it more viable to use the PyPI sdist for building Gentoo, as per #1453 (reply in thread)

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[maresb]

Reverts #1408, but if people find it useful to include the docs in the sdist then we might as well.

[mgorny]

Thanks!

[maresb]

Let's squash-merge when ready