input: Write a python function to split a string into two strings: one containing all the uppercase letters and the other containing all the lowercase letters.
  assert case_split("HelloWorld") == ('HW', 'elloorld')
  assert case_split("Python") == ('P', 'ython')
  assert case_split("aBcDeF") == ('BDF', 'ace')
output: def case_split(s):
      uppercase = ''.join([char for char in s if char.isupper()])
      lowercase = ''.join([char for char in s if char.islower()])
      return (uppercase, lowercase)