input: Write a python function to retrieve the keys of a dictionary.
  assert get_keys({'a': 1, 'b': 2, 'c': 3}) == ['a', 'b', 'c']
  assert get_keys({'x': 10, 'y': 20, 'z': 30}) == ['x', 'y', 'z']
  assert get_keys({'key1': 'value1', 'key2': 'value2'}) == ['key1', 'key2']
output: def get_keys(d):
      return list(d.keys())