input: Write a function to detect a vowel in a given string. If a vowel is found, return the vowel character; otherwise, return None. The function should be case-insensitive.
  def vowelCheck(inputString): 
      vowels = ['a','e','i','o','u','A','E','I','O','U'] 
      for x in inputString: 
          if x in vowels: 
              return x
  assert vowelCheck("Hello World")=='e'
  assert vowelCheck("Python Programming")=='o'
  assert vowelCheck("Rhythm")==None
output: def vowelCheck(inputString): 
      vowels = ['a','e','i','o','u','A','E','I','O','U'] 
      for x in inputString: 
          if x in vowels: 
              return x