Insert dashes every 3 characters

Write a function insert_dashes that receives a string and inserts dashes every 3 characters. Example:

insert_dashes('Hello World')  # 'Hel-lo -Wor-ld'
insert_dashes('AwesomePythonRmotr')  # 'Awe-som-ePy-tho-nRm-otr-'
import unittest


class InsertDashesTestCase(unittest.TestCase):

    def test_with_large_string(self):
        self.assertEqual(insert_dashes('Python is Awesome'),
                         'Pyt-hon- is- Aw-eso-me')

    def test_with_short_string(self):
        self.assertEqual(insert_dashes('Python'), 'Pyt-hon-')