Package 19.0

Hi,

I was working on packaging 0.19 needed for postgresql-12, but it will fail for python3.8 and probably the new cython as well. Upstream just seems not ready for it yet.

Therefore this MR is just a step on the way, but not intended to be uploaded as-is without further fixes.

Details see the update that I'll submit any minute to the open bug 943832.

Note: I'll also push MRs for pristine-tar and upstream branches along this.