input: def filter_vowels(string: str) -> str:
      """ Remove all vowels from a given string.
      >>> filter_vowels("hello")
      'hll'
      >>> filter_vowels("AEIOUaeiou")
      ''
      """
output:     vowels = "AEIOUaeiou"
      return ''.join([char for char in string if char not in vowels])