Mark typing.TYPE_CHECKING as Final (#13180)

Author: brianschubert

stdlib/typing.pyi

@@ -757,7 +757,7 @@ class MutableMapping(Mapping[_KT, _VT]):
 
 Text = str
 
-TYPE_CHECKING: bool
+TYPE_CHECKING: Final[bool]
 
 # In stubs, the arguments of the IO class are marked as positional-only.
 # This differs from runtime, but better reflects the fact that in reality