rust-cpython: removed now useless py_set() conversion In rust-cpython 0.3.0, HashSets implement the appropriate ToPythonObject, we can therefore get rid of this hacky conversion. There still remains an inefficiency in `MissingAncestors.bases()`: we have to clone, because `to_py_object()` requires full ownership. However: - the only use case outside of unit tests used to be from `setdiscovery.partialdiscovery` which is now fully implemented in Rust. - it's not worse than what `py_set()` used to do Differential Revision: https://phab.mercurial-scm.org/D7120