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

it's not safe to remove brackets that are followed by an operator with
lower precedence than PERCENT

src/black/__init__.py

@@ -3238,7 +3238,8 @@ class StringParenStripper(StringTransformer):
     Requirements:
         The line contains a string which is surrounded by parentheses and:
             - The target string is NOT the only argument to a function call).
-            - The RPAR is NOT followed by an attribute access (i.e. a dot).
+            - The RPAR is NOT followed by an operator with higher precedence
+              than PERCENT.
 
     Transformations:
         The parentheses mentioned in the 'Requirements' section are stripped.
@@ -3287,8 +3288,14 @@ 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 anything with higher
+                # precedence than PERCENT
+                if is_valid_index(next_idx + 1) and LL[next_idx + 1].type in {
+                    token.DOUBLESTAR,
+                    token.LSQB,
+                    token.LPAR,
+                    token.DOT,
+                }:
                     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")