Build wheels from sdist in CI

[webknjaz]

This patch integrates re-actors/checkout-python-sdist in place of actions/checkout.
It allows making sure that the sdists are downstream-friendly by not relying on Git
checkouts for making wheels. This is how pip makes wheels when there's no matching
built distributions.

[nitzmahone]

Implemented by #79

[webknjaz]

@nitzmahone partially, right?

[nitzmahone]

@webknjaz All the wheels are being built from the same-run sdist artifact now, unless I missed something? The only checkouts now are the sdist job itself and the matrix generator jobs, since those need access to the (currently) in-repo custom action. Once that's settled down, I might externalize it (esp since I'm using it in PyYAML too), but want to see how it holds up in the real world for awhile.

[webknjaz]

@nitzmahone so the thing I noticed is that you unpack the sdist manually. However, cibuildwheel can do that for you, in a manner that is cleaner and closer to the real-world builds from sdist in the build frontends like pip. I think that's something to implement still.

[webknjaz]

@nitzmahone this is what I had in mind: #82.