input: Write a python function to count the occurrences of a specific character in a string.
  assert count_occurrences('hello world', 'l') == 3
  assert count_occurrences('banana', 'a') == 3
  assert count_occurrences('python', 'p') == 1
  assert count_occurrences('mississippi', 's') == 4
output: def count_occurrences(s: str, char: str) -> int:
      return s.count(char)