input: Write a function to replace all occurrences of a character in a string with the provided replacement character.
  
  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("Hello World!", 'o', '*') == "Hell* W*rld!"
  assert replace_all_occurrences("Hello World!", ' ', '_') == "Hello_World!"
  assert replace_all_occurrences("Hello World!", 'z', 'Y') == "Hello World!"
  assert replace_all_occurrences("", 'a', 'b') == ""
output: def replace_all_occurrences(str, char, replacement):
    return str.replace(char, replacement)