Write a Python script that counts and prints the number of A's in a DNA
sequence. The input is one sequence in FASTA format in a file called
"dna.txt".

For example if the file contains 

>human
ACCGT

then the output of the program should be 1. Your program should work for 
any sequence and not just the one in the example.

Submit a hardcopy in class on Sept 9th.