input: Write a python function to extract a substring from a given string using given start and end indices.
  assert extract_substring('Python is fun', 7, 11) == 'is f'
  assert extract_substring('Hello World!', 0, 5) == 'Hello'
  assert extract_substring('Data Science', 5, 12) == 'Science'
output: def extract_substring(s: str, start: int, end: int) -> str:
      return s[start:end]