Write a Python script that prints the names of the ongest, shortest, and median
sequence length of a set of sequences given in FASTA format. Assume
that the file always has an odd number of sequences.

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

>human
ACCGTA
>mouse
ACCGTAACCTTG
>cat
TCCTATAA
>dog
ACC
>rat
ACACAGTTTGCGCGCGCGGGG
>sheep
GCTTTAAAAAGGCAA
>cow
ATGTA

then the program should print 

rat, dog, cat

Your program should work for any input file and not just this example. Submit a 
hardcopy in class on Oct 28th.