Add `NoReturn` to `__all__` (#983)

Came up in python/typeshed#6619
diff --git a/src/typing_extensions.py b/src/typing_extensions.py
index 9f1c7aa..455e319 100644
--- a/src/typing_extensions.py
+++ b/src/typing_extensions.py
@@ -82,6 +82,7 @@
     'TypeAlias',
     'TypeGuard',
     'TYPE_CHECKING',
+    'NoReturn',
 ]
 
 if PEP_560: