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

src/betterproto/plugin/models.py

@@ -170,6 +170,8 @@ class ProtoContentBase:
     comment_indent: int = 4
     parent: Union["betterproto.Message", "OutputTemplate"]
 
+    __dataclass_fields__: Dict[str, object]
+
     def __post_init__(self) -> None:
         """Checks that no fake default fields were left as placeholders."""
         for field_name, field_val in self.__dataclass_fields__.items():