bpo: update api submit code for bpo rewrite
Rename .sr.ht to .build.postmarketos.org, because the submit script is talking to the bpo server and may not even be using sourcehut if running with the local job service.
Update the script to work with the new API. See the build.postmarketos.org git repository for details: https://gitlab.com/postmarketOS/build.postmarketos.org/
Creating as merge request, so I can see if CI passes (flake 8 might complain about python code), then merging directly. This is only used by the bpo code, and I'm the only one working on that right now, so it should be fine to do it that way.