Write letter_grade(score) turning a mark out of 100 into a letter.
90 and above is "A", 80 to 89 is "B", 70 to 79 is "C", 60 to 69 is "D", and anything below 60 is "F".
A score outside 0 to 100 is not a mark at all, so return None for it.
Mind the boundaries: exactly 90 is an A, and exactly 89 is a B.