rust-pyo3: dagop submodule implementation This is the first demonstration that the passing of objects through the Python interpreter from `rustext` to `pyo3_rustext` actually works. In the Python tests, we conflate the presence of the `pyo3_rustext` package with that of `rustext`, as we do not plan to support building one and not the other (we hope to convert fully to PyO3 soon). The skipping is actually done by the base test class. The implementation of `rank` is as trivial as it was in `hg-cpython`, yet it required to be able to convert exceptions from `vcsgraph`.