Extend dict keys to allow for string literals? #76

from typing import Literal
from msgspec import Struct, json

AorB = Literal["a", "b"]

class Test(Struct):
    d: dict[AorB, float]

data = Test(d={'a': 1.0, 'b': 2.0})
encoded = json.encode(data)
print(encoded)

decoded = json.decode(encoded, type=Test) # TypeError: JSON doesn't support dicts with non-string keys - type `dict[typing.Literal['a', 'b'], float]` is not supported