How can I convert a gpa calculator to use upper and lower case grade arguments?

Question

I am creating a GPA calculator which takes in 4 grades and gives the GPA. I want it to be able to use upper case letters 'A' and lower case letters 'a' and so on through 'F' 'f'. Right now it will only use upper case letters. How can I convert it to use both without changing the dictionary?

import sys
def gpa_calculator(grade1, grade2, grade3, grade4):
    points = 0
    i = 0
    grade_c ={'A':4.0, 'A-':3.66, 'B+':3.33, 'B':3.0,
      'B-':2.66,'C+':2.33,'C':2.0,'C-':1.66,'D+':1.33,'D':1.00,'D-':.66,'F':0.00}

    if grades != []:
        for grade in grades:
            points += grade_c[grade]
        gpa = points / len(grades)
        return gpa
    else:
        return None

    grade1 = sys.argv[1]
    grade2 = sys.argv[2]
    grade3 = sys.argv[3]
    grade4 = sys.argv[4]

    grades = grade1, grade2, grade3, grade4
    grades = gpa_calculator(grade1, grade2, grade3, grade4)
    print('My GPA is',(grades))