def METHOD_NAME(self) -> str:
    """Runtime of the build system.
    Value should be a valid Lambda Function runtime
    (https://docs.aws.amazon.com/lambda/latest/dg/lambda-runtimes.html).
    """
    if self._runtime_from_docker:
        return self._validate_runtime(self._runtime_from_docker)
    return self._validate_runtime(
        f"python{self.pip.python_version.major}.{self.pip.python_version.minor}"
    )