Please give me the code for the following function in Python 3:

Function name : calc_grades

Parameters : s_grades (list)

Return Type : dict

Description : This function should return a dictionary that maps the students name to their final letter grade. The final grade is calculated by taking 0.25*(Test 1 + Test 2 + Test 3 + Test 4) . When determining the letter grade, use the following grade cut offs: A: 90.0, B: 80.0, C: 70.0, D: 60.0

s_grades = [ [ 'Student', 'Test 1', 'Test 2', 'Test 3', 'Test 4'], [ 'Ed', '100', '90', '80', '90'], [ 'Jay', '88', '99', '111', '92'], [ 'Mary', '45', '56', '67', '54'] ]

you may assume that the format of s_grades will stay the same (always have 4 exams) except the number of students may change

Test Case :

>>> calc_grades(s_grades)

{ 'Ed': 'A', 'Jay': 'A', 'Mary': 'F' }