Add responsive fork of gpodder

This MR emerged from the discussion in #872 (closed) and adds gpodder based on @tpikonen's hdy branch. Three dependencies not present in Alpine were needed: mygpoclient, podcastparser and pytest-httpserver.

So far, I only tested this in Qemu. Some of the dialogs don't currently fit into the screen size yet but the main window appears to be usable.

I also tried to use it with Lollypop as media player (gpodder uses xdg-open to launch it) but the latter is currently broken due to https://gitlab.alpinelinux.org/alpine/aports/-/issues/12211.