input: Write a python function to extract all keys from a dictionary and return them as a list.
  assert extract_keys({'name': 'John', 'age': 56, 'city': 'New York'}) == ['name', 'age', 'city']
  assert extract_keys({'a': 1, 'b': 2, 'c': 3}) == ['a', 'b', 'c']
  assert extract_keys({}) == []
output: def extract_keys(d):
      return list(d.keys())