Right now we don't really use Literal types anywhere, but we should.
For example, see the following signature:

    @classmethod
    def from_bytes(
        cls: Type[T],
        data: bytes,
        protocol: str = 'protobuf',
        compress: Optional[str] = None,
    ) -> T:
        """Build Document object from binary bytes

        :param data: binary bytes
        :param protocol: protocol to use. It can be 'pickle' or 'protobuf'
        :param compress: compress method to use
        :return: a Document object
        """

Here, protocol can only be 'pickle' or 'protobuf', so Literal['pickle', 'protobuf'] would be the most suitable type hint.
Changing that will require multiple other changes in order to keep mypy happy, since other code relies on protocol being str.

This issue is about adjusting the above, and identifying and fixing other similar instances (if they exists).