Misc cleanup, see commit body (#227)

- Enable oneof_enum test case that passes now (removed the xfail)
- Switch from toml to tomlkit as a dev dep for better toml support
- upgrade poethepoet to latest stable release
- use full table format for poe tasks to avoid long lines in pyproject.toml
- remove redundant _WrappedMessage class
- fix various Mypy warnings
- reformat some comments for consistent line length