Write a Python script that prints the longest, shortest, and mean
sequence length of a set of sequences given in FASTA format. 

For example if the input in the file dna.txt is

>A
ACCGTA
>B
ACCGTAACCTTG
>C
TCCTATAA
>D
ACC
>E
ACACAGTTTGCGCGCGCGGGG
>F
GCTTTAAAAAGGCAA
>G
ATGTA

then the program should print 

21, 3, 10

Your program should work for any input file and not just this example. Submit a 
hardcopy in class on Sept 20th.