input: Write a function to replace all occurrences of a given character in a string with the provided replacement character. The function 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("Hello World!", 'o', 'Y')=="HellY WYrld!"
  assert replace_all_occurrences("Python Programming", 'P', 'Q')=="Qython Qrogramming"
  assert replace_all_occurrences("Python Programming", 'g', 'Z')=="Python ProZramminZ"
  assert replace_all_occurrences("", 'a', 'b')==""
output: def replace_all_occurrences(str, char, replacement):
    return str.replace(char, replacement)