Fix the flatpak build; add script to update the python requirements

Fixes #242 (closed)

This does a few things:

• Explicitly update lxml to its latest version. I don't know why pip is picking up 4.9.0, which doesn't build.

• Add a ci/update-python-requiremente script. It runs flatpak-pip-generator and manually adds the special versions of python3-puzpy and python3-xdfile that we need.

• Update RELEASING.md to that effect.

Note that the container image is updated to have a newer rustc, and a checkout of flatpak-builder-tools in /usr/local.