input: Write a function to replace all occurrences of a given character in a string with the provided replacement character. If the given character is not found in the string, return the original 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("Hello World!", 'o', 'Y')=="HellY WYrld!"
  assert replace_all_occurrences("Hello World!", 'z', 'X')=="Hello World!"
  assert replace_all_occurrences("", 'o', 'Y')==""
  assert replace_all_occurrences("Hello World!", '', 'X')=="Hello World!"
output: def replace_all_occurrences(str, char, replacement):
    return str.replace(char, replacement)