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

tests/inputs/config.py

@@ -1,7 +1,6 @@
 # Test cases that are expected to fail, e.g. unimplemented features or bug-fixes.
 # Remove from list when fixed.
 xfail = {
-    "oneof_enum",  # 63
     "namespace_keywords",  # 70
     "namespace_builtin_types",  # 53
     "googletypes_struct",  # 9