Build an abi3 version of this package

[Luthaf]

This package is completely independent of the python version (we don't use any symbol from libpython), but dependent on the os/arch used to build the package (we record the path to the shared library to load).

We should still be able to leverage abi3 packages to only build it once per os/arch combination, and remove the need to build the package once per Python version.