shells/xonsh: Update to 0.23.3
- Temporarily switch to GitHub as release tarball on PyPI is missing files necessary for unit tests
- Move prompt-toolkit to RUN_DEPENDS as it is required for the interactive shell
- Thanks to Andy Kipp for assisting in improving and fixing unit tests and improving FreeBSD support
ChangeLog: https://github.com/xonsh/xonsh/releases/tag/0.23.3