Get grade letter

Define a function get_grade_letter that receives a score and you should return:

'A' if the score is 90 or above

'B' if the score is 80 to 89

'C' if the score is 70 to 79

'D' if the score is 60 to 69

'F' if the score is less than 60

Examples:

>>> get_grade_letter(93)
'A'
>>> get_grade_letter(80)
'B'
>>> get_grade_letter(75)
'C'
>>> get_grade_letter(67)
'D'
>>> get_grade_letter(42)
'F'
import unittest


class GetGradeLetterTestCase(unittest.TestCase):

    def test_A_score(self):
        self.assertEqual(get_grade_letter(93), 'A')

    def test_B_score(self):
        self.assertEqual(get_grade_letter(80), 'B')

    def test_C_score(self):
        self.assertEqual(get_grade_letter(75), 'C')

    def test_D_score(self):
        self.assertEqual(get_grade_letter(67), 'D')

    def test_F_score(self):
        self.assertEqual(get_grade_letter(42), 'F')