Write a Python program that computes a random traceback matrix for
a pair of sequences in FASTA format (in dna.fasta). The number of 
rows and columns is given by the length of the second and first 
sequence respectively plus one. For example if dna.fasta is

>human
ACCC
>mouse
AG

then your matrix T would have 3 rows and 5 columns. The first row
contains 'L' and the first column 'U'. Remaining entries of the 
matrix contain 'U', 'L', or 'D' selected randomly.

Submit hardcopy in class on Feb 7th.