input: Write a python function to find and return words containing the letter 'a' in a given string.
  assert find_words_with_a('This is an example of a string to test.') == ['an', 'example', 'a', 'string', 'test.']
  assert find_words_with_a('A quick brown fox jumps over the lazy dog.') == ['A', 'lazy']
  assert find_words_with_a('Hello world!') == []
output: def find_words_with_a(s):
      return [word for word in s.split() if 'a' in word or 'A' in word]