Describe the bug

We recently added --skip-magic-trailing-comma to black. When running with this option, we'd like it to always remove ugly unnecessary trailing commas that plagued some earlier versions of black, but it doesn't do this in the case of subscripts.

To Reproduce

(venv) ~/tmp/venv λ cat test.pyi
Tuple[A, B,]

def f(a, b,): ...
(venv) ~/tmp/venv λ black -l 130 test.pyi --skip-magic-trailing-comma --check --diff
--- test.pyi	2021-06-01 07:57:26.995533 +0000
+++ test.pyi	2021-06-01 07:58:31.094021 +0000
@@ -1,3 +1,3 @@
 Tuple[A, B,]
 
-def f(a, b,): ...
+def f(a, b): ...
would reformat test.pyi
Oh no! 💥 💔 💥
1 file would be reformatted.

Expected behavior

If the subscript expression has more than one element, we should remove the extra trailing comma.