[BUG] Functions in `LetterCase` aren't members of the enum

[benberryallwood]

Description

The attributes in the LetterCase enum are functions, so they're being registered as methods rather than enum members.

Mypy doesn't recognise this as an issue, but Pyright shows errors when trying to do, e.g., @dataclass_json(letter_case=LetterCase.CAMEL):

Argument of type "(string: Unknown) -> (Unknown | Literal[''])" cannot be assigned to parameter "letter_case" of type "LetterCase | None" in function "dataclass_json"   Type "(string: Unknown) -> (Unknown | Literal[''])" is not assignable to type "LetterCase | None"     "function" is not assignable to "LetterCase"     "function" is not assignable to "None"

Code snippet that reproduces the issue

from dataclasses_json import LetterCase

assert isinstance(LetterCase.CAMEL, LetterCase)

Describe the results you expected

LetterCase.CAMEL should be an instance of LetterCase, so this assertion should pass

Python version you are using

Python 3.10.13

Environment description

attrs==23.1.0
black==22.12.0
certifi==2023.7.22
charset-normalizer==3.2.0
click==8.1.7
coverage==7.2.7
docstring-parser==0.15
exceptiongroup==1.1.3
falcon==2.0.0
flake8==5.0.4
ghp-import==2.1.0
gitdb==4.0.10
GitPython==3.1.32
hug==2.6.1
hypothesis==6.79.4
idna==3.4
importlib-metadata==4.2.0
iniconfig==2.0.0
Jinja2==3.1.2
livereload==2.6.3
Mako==1.2.4
Markdown==3.3.4
MarkupSafe==2.1.3
marshmallow==3.19.0
mccabe==0.7.0
mergedeep==1.3.4
mkdocs==1.2.4
mkdocs-material==7.3.0
mkdocs-material-extensions==1.1.1
mypy==1.4.1
mypy-extensions==1.0.0
packaging==23.1
pathspec==0.11.2
pdocs==1.2.0
platformdirs==3.10.0
pluggy==1.2.0
portray==1.7.0
pycodestyle==2.9.1
pyflakes==2.5.0
Pygments==2.16.1
pymdown-extensions==7.1
pytest==7.4.0
pytest-cov==2.12.1
python-dateutil==2.8.2
PyYAML==6.0.1
pyyaml_env_tag==0.1
requests==2.31.0
simplejson==3.19.1
six==1.16.0
smmap==5.0.0
sortedcontainers==2.4.0
toml==0.10.2
tomli==2.0.1
tornado==6.2
typing-inspect==0.9.0
typing_extensions==4.7.1
urllib3==2.0.4
watchdog==3.0.0
yaspin==0.15.0
zipp==3.15.0