Python's str.isAlpha(), reimplemented without Unicode support
In this answer I am assuming that high performance is not needed

In general that approach is okay but I do not think that code is fulfilling the requirements. For example 

ascii_letters("A2") == True
ascii_letters("") == None


I propose following steps:


Fix your algorithm (some unit tests would be great!) 
Think about if is this really necessary to check whether the letter is in set of containing both lower and upper case letters.
Next, think about iteration over whole string: is this needed? There are functions in python which can check logical condition on whole list.
Why do you need to include string ? There is no need to override already existing variables. 
Try to do pythonic one liner using all points above.