Update on "[JIT] add support for torch.jit.Final in python 3.6"

Python added support for typing.Final in 3.8, and there exists a backport for 3.7 and 3.6 in typing_extensions.Final

The 3.6 version of typing_extensions.Final doesn't support the `getattr(x, '__origin__')` to determine if an object is of the Final type while 3.7 does. This PR allows for the Final type of an object to be discoverable in 3.6


Differential Revision: [D24739402](https://our.internmc.facebook.com/intern/diff/D24739402)

[ghstack-poisoned]

torch/_jit_internal.py

@@ -717,8 +717,7 @@ def is_rref(ann):
 
 def is_final(ann):
     return ann.__module__ in {'typing', 'typing_extensions'} and \
-        (getattr(ann, '__origin__', None) is Final \
-            or isinstance(ann, type(Final)))
+        (getattr(ann, '__origin__', None) is Final or isinstance(ann, type(Final)))
 
 # allows BroadcastingList instance to be subscriptable
 class BroadcastingListCls(object):