Finish finalizes a buffer, pointing to the given `rootTable`.

def __Finish(self, rootTable, sizePrefix):
        """Finish finalizes a buffer, pointing to the given `rootTable`."""
        N.enforce_number(rootTable, N.UOffsetTFlags)
        prepSize = N.UOffsetTFlags.bytewidth
        if sizePrefix:
            prepSize += N.Int32Flags.bytewidth
        self.Prep(self.minalign, prepSize)
        self.PrependUOffsetTRelative(rootTable)
        if sizePrefix:
            size = len(self.Bytes) - self.Head()
            N.enforce_number(size, N.Int32Flags)
            self.PrependInt32(size)
        self.finished = True
        return self.Head()