input: Write a function to expand a given range string into a list of integers.
  assert expand_range("1-5") == [1, 2, 3, 4, 5]
  assert expand_range("10-15") == [10, 11, 12, 13, 14, 15]
  assert expand_range("7-9") == [7, 8, 9]
output: def expand_range(range_str):
      start, end = map(int, range_str.split('-'))
      return list(range(start, end + 1))