[typing_extensions] Use unconstrained type var for final (#15622)

This matches `typing.final`.

Author: zedzhen

stdlib/typing_extensions.pyi

@@ -220,7 +220,7 @@ def runtime_checkable(cls: _TC) -> _TC: ...
 runtime = runtime_checkable
 Final: _SpecialForm
 
-def final(f: _F) -> _F: ...
+def final(f: _T) -> _T: ...
 def disjoint_base(cls: _TC) -> _TC: ...
 
 Literal: _SpecialForm