input: Write a python function to convert a list to a dictionary with keys from the list and values as None.
  assert list_to_dict([1, 2, 3, 4]) == {1: None, 2: None, 3: None, 4: None}
  assert list_to_dict(['a', 'b', 'c']) == {'a': None, 'b': None, 'c': None}
  assert list_to_dict([True, False, None]) == {True: None, False: None, None: None}
output: def list_to_dict(lst):
      return {key: None for key in lst}