FWIW, ruff sees your line comment and keeps each item on its line. It only adds a trailing comma and additional space.

It also does not collapse lines when there’s a trailing comma.

Your output seems to be from black. IMO it’s insane to collapse lines when there are line comments.

  $ uvx ruff format --diff formatting.py
  --- formatting.py
  +++ formatting.py
  @@ -1,8 +1,4 @@
  -no_comma  = {
  -    "x": 3,
  -    "y": 42,
  -    "z": 2
  -}
  +no_comma = {"x": 3, "y": 42, "z": 2}
  
  with_comma = {
       "x": 3,
  @@ -12,6 +8,6 @@
  
  comment = {
       "x": 3,
  -    "y": 42, # Answer to the Ultimate Question!
  -    "z": 2
  +    "y": 42,  # Answer to the Ultimate Question!
  +    "z": 2,
   }
  
  1 file would be reformatted

(I intentionally switched to double quotes since that really is a stylistic choice in Python, you can escape in both, and if you use double quotes inside of single quotes ruff leaves it as-is)