Fixes @final and ParamSpec integration

[sobolevn]

I am not 100% sure that this is correct though.

Closes #12033

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

steam.py (https://github.com/Gobot1234/steam.py)
- steam/ext/commands/commands.py:744: error: Overloaded function implementation cannot produce return type of signature 2  [misc]
- steam/ext/commands/commands.py:813: error: Overloaded function implementation cannot produce return type of signature 2  [misc]
- steam/ext/commands/commands.py:919: error: Overloaded function implementation cannot produce return type of signature 2  [misc]
- steam/ext/commands/commands.py:986: error: Overloaded function implementation cannot produce return type of signature 2  [misc]

[Akuli]

This reproduces the problem without @final:

from typing import Generic, Any, TypeVar, Callable
from typing_extensions import ParamSpec

_P = ParamSpec("_P")
_F = TypeVar("_F", bound=Callable[..., Any])

def f(x: _F) -> _F: pass

@f
class OnlyParamSpec(Generic[_P]):
    pass

It seems to me that a correct fix would require more work. Currently is_callable_compatible() solves and substitutes type variables, if there are any. Maybe it should also replace ParamSpecs with something very permissive, e.g. Callable[..., Any]? Maybe this should be fixed in unify_generic_callable(), the function that solves and substitutes TypeVars? Even if we ignore all this, we should at least preserve the symmetry when allow_partial_overlap=True is given.

(I haven't ever contributed anything non-trivial to mypy, but I have lately poked around mypy with pdb a bit)

[sobolevn]

Closing in favor of #12096