input: Write a python function to search for a 4-digit year in a given string.
  assert search_year("This movie was released in 1980") == "1980"
  assert search_year("The car model from 2021 is very popular") == "2021"
  assert search_year("No year mentioned here") == None
output: def search_year(s: str) -> str:
      import re
      match = re.search(r'\b\d{4}\b', s)
      return match.group(0) if match else None