input: Write a function that replaces all occurrences of a specified character in a given string with another specified character. It should return the modified string.
  
  def replace_all_occurrences(str, char, replacement):
      return str.replace(char, replacement)
  
  assert replace_all_occurrences("Hello World!", 'l', 'X') == "HeXXo WorXd!"
  assert replace_all_occurrences("Python is fun", 'n', 'm') == "Pythom is fum"
  assert replace_all_occurrences("coffee", 'f', 't') == "cootee"
  assert replace_all_occurrences("awesome", 'e', 'a') == "awasoma"
  assert replace_all_occurrences("", 'a', 'b') == ""
output: def replace_all_occurrences(str, char, replacement):
      return str.replace(char, replacement)