input: Write a python function to remove all vowels from a given string.
  assert remove_vowels("Hello, World!") == "Hll, Wrld!"
  assert remove_vowels("This is a test.") == "Ths s  tst."
  assert remove_vowels("Aeiou are vowels.") == "r vwls."
output: def remove_vowels(s):
      vowels = "aeiouAEIOU"
      return ''.join(char for char in s if char not in vowels)