$4.$ Implement letter$\_$grades

Read the specification of the function letter$\_$grades in the module funcs. Implement this function according to its specification. Once again, the inputs to this function must be dictionaries that have netids as keys and numbers as values. Look at the test cases in tests.py if you are unsure.

When you have implemented the function, test your answer with the test script. You should now pass all tests.