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!", 'o', 'X')=="HellX WXrld!"
  assert replace_all_occurrences("Python is fun", 'P', 'p')=="python is fun"
  assert replace_all_occurrences("A quick brown fox", ' ', '-')=="A-quick-brown-fox"
  assert replace_all_occurrences("Mississippi", 'i', 'a')=="Massaassa"
  assert replace_all_occurrences("Hello Hello", 'H', 'h')=="hello hello"
  assert replace_all_occurrences("", 'x', 'y')==""
output: def replace_all_occurrences(str, char, replacement):
    return str.replace(char, replacement)