Generate a random string of `BLOCK_SIZE` length.

def _random_block():
    """
    Generate a random string of `BLOCK_SIZE` length.
    """
    # TODO: Use a better RNG than random.randint
    random_number = random.randint(0, DISCRETE_VALUES)
    random_string = _to_base36(random_number)
    return _pad(random_string, BLOCK_SIZE)