def is_prime(n: int) -> bool:
    """
    >>> is_prime(10)
    True
    >>> is_prime(17)
    True
    >>> is_prime(21)
    False
    """
    for el in range(2, n):
        if n % el == 0:
            return False
    return True


if __name__ == "__main__":
    import doctest

    doctest.testmod()