Fix EnvBaseMixins issues when subclassing with other generics

[rpmcginty]

Details

fixes an issue in aibs-informatics-aws-lambda

env_base = None, env_base_class = <class 'aibs_informatics_aws_lambda.handlers.demand.model.PrepareDemandScaffoldingRequest'>

    def get_env_base(
        env_base: Optional[Union[str, E]] = None,
        env_base_class: Optional[Type[Union[E, EnvBase]]] = None,
    ) -> Union[E, EnvBase]:
        """Will look for the env_base as an environment variable."""
        env_base_cls: Type[E] = env_base_class or EnvBase  # type: ignore[assignment]
        if env_base:
            if isinstance(env_base, env_base_cls):
                return env_base
            else:
                return env_base_cls(env_base)
>       return env_base_cls.from_env()
E       AttributeError: type object 'PrepareDemandScaffoldingRequest' has no attribute 'from_env'

[rpmcginty]

verified make release passes