input: Write a python function to extract all the capital letters from a given string.
  assert extract_capitals("Hello World!") == "HW"
  assert extract_capitals("This is a Test String") == "TTSS"
  assert extract_capitals("no capitals here") == ""
  assert extract_capitals("MIXED CaSe LeTTers") == "MIXEDCSTT"
output: def extract_capitals(s):
      return ''.join([char for char in s if char.isupper()])