community/py3-qt6: check python dependency info in prepare()

This commit introduces the _check_dep() function which is used in prepare() to read python dependency information from the pyproject.toml and PKG-INFO files, and check it against the versions of the dependencies installed.

Yes its hacky, but I keep forgetting that riverbank doesn't put any useful dependency version information in the release notes. They seem to rely entirely on their python toolchain to get it right, which does not work with our build process. With this code I will at least be pointed in the right direction the next time the dependency chain breaks.

Of course, if anyone else has a better way to manage these I would love to hear it!