PyOxidizer 0.18.0

Release notes at https://pyoxidizer.readthedocs.io/en/pyoxidizer-0.18.0/pyoxidizer_history.html#version-0-18-0.

We would like to thank @davidhewitt and the other maintainers of the PyO3 project. This PyOxidizer release, its switch to PyO3, and the benefits that switch provides - and will provide - would not be possible without your willingness to support PyOxidizer's somewhat esoteric use cases. For your patience and support, we thank you.