Simple:
>>> import string
>>> string.ascii_letters
'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
>>> import random
>>> random.choice(string.ascii_letters)
'j'
string.ascii_letters
returns a string containing the lower case and upper case letters according to the current locale.
random.choice
returns a single, random element from a sequence.