Improve syntax error when brackets (parentheses) are mismatched

The syntax error for mismatched brackets uses a caret to point to the closing bracket. But sometimes it is the opening bracket which is wrong. We should point a caret at both.

For example, this simple expression points to the closing square bracket:

>>> obj{"some long expression"]
  File "<stdin>", line 1
    obj{"some long expression"]
                              ^
SyntaxError: closing parenthesis ']' does not match opening parenthesis '{'

But the real cause of the issue is the opening brace. It would be helpful to point at that as well:

    obj{"some long expression"]
       ^                      ^
SyntaxError: closing parenthesis ']' does not match opening parenthesis '{'

This is especially helpful in complex expressions with many mixed parentheses:

>>> obj[func(x[store[arg, a[i], b[j])])[y]]
  File "<stdin>", line 1
    obj[func(x[store[arg, a[i], b[j])])[y]]
                                    ^
SyntaxError: closing parenthesis ')' does not match opening parenthesis '['

The reader has to carefully match up brackets by hand to see where the opening bracket is. A second caret would make that instantaneous:

    obj[func(x[store[arg, a[i], b[j])])[y]]
                    ^               ^
SyntaxError: closing parenthesis ')' does not match opening parenthesis '['

Start by locating the parser or syntax-error diagnostic code that handles mismatched brackets, then find its existing tests for the reported examples. Update the behavior so the diagnostic points to both the opening and closing brackets, and confirm the displayed caret positions for mixed parentheses.