Strip trailing commas in subscripts with -C

Fixes psf#2296, psf#3204

src/black/lines.py

@@ -273,6 +273,8 @@ def has_magic_trailing_comma(
         - it's not a single-element subscript
         Additionally, if ensure_removable:
         - it's not from square bracket indexing
+            - specifically, single-element square bracket indexing with
+              Preview.skip_magic_trailing_comma_in_subscript
         """
         if not (
             closing.type in CLOSING_BRACKETS
@@ -301,8 +303,22 @@ def has_magic_trailing_comma(
 
             if not ensure_removable:
                 return True
+
             comma = self.leaves[-1]
-            return bool(comma.parent and comma.parent.type == syms.listmaker)
+            if comma.parent is None:
+                return False
+            if Preview.skip_magic_trailing_comma_in_subscript in self.mode:
+                return bool(
+                    comma.parent.type != syms.subscriptlist
+                    or closing.opening_bracket is None
+                    or not is_one_sequence_between(
+                        closing.opening_bracket,
+                        closing,
+                        self.leaves,
+                        brackets=(token.LSQB, token.RSQB),
+                    )
+                )
+            return comma.parent.type == syms.listmaker
 
         if self.is_import:
             return True

src/black/mode.py

@@ -151,6 +151,7 @@ class Preview(Enum):
     remove_block_trailing_newline = auto()
     remove_redundant_parens = auto()
     string_processing = auto()
+    skip_magic_trailing_comma_in_subscript = auto()
 
 
 class Deprecated(UserWarning):

tests/data/preview/skip_magic_trailing_comma.py

@@ -0,0 +1,34 @@
+# We should not remove the trailing comma in a single-element subscript.
+a: tuple[int,]
+b = tuple[int,]
+
+# But commas in multiple element subscripts should be removed.
+c: tuple[int, int,]
+d = tuple[int, int,]
+
+# Remove commas for non-subscripts.
+small_list = [1,]
+list_of_types = [tuple[int,],]
+small_set = {1,}
+set_of_types = {tuple[int,],}
+
+# Except single element tuples
+small_tuple = (1,)
+
+# output
+# We should not remove the trailing comma in a single-element subscript.
+a: tuple[int,]
+b = tuple[int,]
+
+# But commas in multiple element subscripts should be removed.
+c: tuple[int, int]
+d = tuple[int, int]
+
+# Remove commas for non-subscripts.
+small_list = [1]
+list_of_types = [tuple[int,]]
+small_set = {1}
+set_of_types = {tuple[int,]}
+
+# Except single element tuples
+small_tuple = (1,)

tests/test_format.py

@@ -153,6 +153,11 @@ def test_preview_docstring_no_string_normalization() -> None:
     assert_format(source, expected, mode)
 
 
+def test_skip_magic_trailing_comma_in_subscript() -> None:
+    source, expected = read_data("preview", "skip_magic_trailing_comma")
+    assert_format(source, expected, mode=black.Mode(magic_trailing_comma=False, preview=True))
+
+
 def test_long_strings_flag_disabled() -> None:
     """Tests for turning off the string processing logic."""
     source, expected = read_data("miscellaneous", "long_strings_flag_disabled")