input: Write a function to capitalize the first letter of a string.
  assert capitalize_first_letter("hello world") == "Hello world"
  assert capitalize_first_letter("test") == "Test"
  assert capitalize_first_letter("python") == "Python"
output: def capitalize_first_letter(s: str) -> str:
      if not s:
          return s
      return s[0].upper() + s[1:]