[Python] Pydantic restrict string to literal values and convert to lower case

How can I restrict the value of a string field to certain values in a case insensitive way?

>>> Animal(species="antelope")
>>> Animal(species="AnTeLoPe")
>>> Animal(species="frog") # ValidationError: `species must be antelope or zebra`


I know that you can restrict a Pydantic field to certain values:

import pydantic
from typing import Literal


class Animal(pydantic.BaseModel):
species: Literal["antelope", "zebra"]


And I know that you can convert input data to lowercase:

class Animal(pydantic.BaseModel):
species: pydantic.constr(to_lower=True)


But I can't see how I would combine these two features so I don't have to spell out the upper-case and lower-case versions of my set of literals:

class Animal(pydantic.BaseModel):
species: Literal["antelope", "Antelope"] # ...etc


I thought using str_to_lower would help, but it doesn't work:

class Animal(pydantic.BaseModel):
model_config = pydantic.ConfigDict(str_to_lower=True)
species: Literal["antelope", "zebra"]

Animal(species="AnTeLope") # ValidationError


I can see a way out of this with regex, but it seems like this is such a common use-case that using regex seems like an inconvenient workaround (what if my allowed values have got regex-ish characters? then I have to escape them)

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