input: Write a python function to mask parts of a string except spaces.
  assert mask_parts_of_string("This is a demo string", '#') == "#### ## # #### ######"
  assert mask_parts_of_string("Hello World", '*') == "***** *****"
  assert mask_parts_of_string("Python Programming", '$') == "$$$$$$ $$$$$$$$$$$$"
output: def mask_parts_of_string(s, mask_char):
      return ''.join(mask_char if c != ' ' else ' ' for c in s)