Build for ABI3 #264

At the moment we build each release for all supported Python version at time of release.

Instead, we could build for the Stable ABI and have one wheel per platform instead of one per platform per python runtime and version.

The main benefit is that when a new Python version is available (beta, rc, released), one can use our existing wheel.

Another benefit is that release build time should be greatly reduced (currently more than 2 hours).