input: Write a python function to count the number of uppercase letters in a string.
  assert uppercase_letter_count("Hello World!") == 2
  assert uppercase_letter_count("PYTHON programming") == 6
  assert uppercase_letter_count("this is a test.") == 0
output: def uppercase_letter_count(s: str) -> int:
      return sum(1 for char in s if char.isupper())