Add trailing comma test case for hugging parens (#3991)

docs/the_black_code_style/future_style.md

@@ -139,6 +139,19 @@ foo([
 ])
 ```
 
+You can use a magic trailing comma to avoid this compacting behavior; by default,
+_Black_ will not reformat the following code:
+
+```python
+foo(
+    [
+        1,
+        2,
+        3,
+    ],
+)
+```
+
 ### Improved multiline string handling
 
 _Black_ is smarter when formatting multiline strings, especially in function arguments,

tests/data/cases/preview_hug_parens_with_braces_and_square_brackets.py

@@ -36,6 +36,14 @@ def foo_square_brackets(request):
     ]
 )
 
+func(
+    [
+        'a',
+        'b',
+        'c',
+    ],
+)
+
 func(  # a
     [  # b
         "c",  # c
@@ -171,6 +179,14 @@ def foo_square_brackets(request):
     "c",
 ])
 
+func(
+    [
+        "a",
+        "b",
+        "c",
+    ],
+)
+
 func([  # a  # b
     "c",  # c
     "d",  # d