input: Write a python function to filter out vowels from a string.
  assert filter_vowels('hello world') == 'hll wrld'
  assert filter_vowels('aeiou') == ''
  assert filter_vowels('Python programming is fun') == 'Pythn prgrmmng s fn'
output: def filter_vowels(s):
      vowels = 'aeiouAEIOU'
      return ''.join([char for char in s if char not in vowels])