don't strip brackets before lsqb (psf#1575)

it's not safe to remove brackets that are followed by a `[`

src/black/__init__.py

@@ -3287,8 +3287,11 @@ def do_match(self, line: Line) -> TMatchResult:
                 and LL[next_idx].type == token.RPAR
                 and not is_empty_rpar(LL[next_idx])
             ):
-                # That RPAR should NOT be followed by a '.' symbol.
-                if is_valid_index(next_idx + 1) and LL[next_idx + 1].type == token.DOT:
+                # That RPAR should NOT be followed by a '.' or a '['
+                if is_valid_index(next_idx + 1) and LL[next_idx + 1].type in {
+                    token.DOT,
+                    token.LSQB,
+                }:
                     continue
 
                 return Ok(string_idx)

tests/data/bracketed_slicing.py

@@ -0,0 +1,3 @@
+("" % a)[0]
+# output
+("" % a)[0]

tests/test_black.py

@@ -487,6 +487,14 @@ def test_slices(self) -> None:
         black.assert_equivalent(source, actual)
         black.assert_stable(source, actual, black.FileMode())
 
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_bracketed_slicing(self) -> None:
+        source, expected = read_data("bracketed_slicing")
+        actual = fs(source)
+        self.assertFormatEqual(expected, actual)
+        black.assert_equivalent(source, actual)
+        black.assert_stable(source, actual, black.FileMode())
+
     @patch("black.dump_to_file", dump_to_stderr)
     def test_comments(self) -> None:
         source, expected = read_data("comments")