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/test_version.py

@@ -1,13 +1,13 @@
 from betterproto import __version__
 from pathlib import Path
-import toml
+import tomlkit
 
 PROJECT_TOML = Path(__file__).joinpath("..", "..", "pyproject.toml").resolve()
 
 
 def test_version():
     with PROJECT_TOML.open() as toml_file:
-        project_config = toml.load(toml_file)
+        project_config = tomlkit.loads(toml_file.read())
     assert (
         __version__ == project_config["tool"]["poetry"]["version"]
     ), "Project version should match in package and package config"