input: Write a function to convert a number to its corresponding month.
  assert num_to_month(1) == 'January'
  assert num_to_month(12) == 'December'
  assert num_to_month(7) == 'July'
output: def num_to_month(num):
      months = {
          1: 'January',
          2: 'February',
          3: 'March',
          4: 'April',
          5: 'May',
          6: 'June',
          7: 'July',
          8: 'August',
          9: 'September',
          10: 'October',
          11: 'November',
          12: 'December'
      }
      return months.get(num, 'Invalid month')