Improve syntax error when brackets (parentheses) are mismatched

[stevendaprano]

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 '['

[pochmann]

In your first example, shouldn't it even only point to the { and not look further? That's also what Python 3.8 still did.