Fixes #16245
Fixes #16248

Unfortunately I was a bit reckless with parentheses, but in my defense
`unify_generic_callable()` is kind of broken for long time, as it can
return "solutions" like ```{1: T`1}```. We need a more principled
approach there (IIRC there is already an issue about this in the scope
of `--new-type-inference`).

(The fix is quite trivial so I am not going to wait for review too long
to save time, unless there will be some issues in `mypy_primer` etc.)

Fixes #16257

Parenthesis strike back. I hope this is the last place where I had put
them wrong.