Write a Python script that prints the number of times each sequence occurs
in sorted descending order from a file given in FASTA format. 

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

>human
ACCGTA
>mouse
ACCGTAACCTTG
>cat
ACCGTA
>dog
ACC
>rat
ACACAGTTTGCGCGCGCGGGG
>sheep
ACCGTAACCTTG
>cow
GCTTTAAAAAGGCAA
>chicken
ACC
>horse
ACCGTA

then the program should print 

ACCGTA: 3
ACCGTAACCTTG: 2
ACC: 2
ACACAGTTTGCGCGCGCGGGG: 1
CGAGAGGAC: 1

Your program should work for any input file and not just this example. Submit a 
hardcopy in class on Oct 4th.