bpo-40618: Disallow invalid targets in augassign and except clauses (GH-20083)

This commit fixes the new parser to disallow invalid targets in the
following scenarios:
- Augmented assignments must only accept a single target (Name,
  Attribute or Subscript), but no tuples or lists.
- `except` clauses should only accept a single `Name` as a target.

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

Lib/test/test_syntax.py

@@ -129,6 +129,18 @@ SyntaxError: cannot assign to __debug__
 Traceback (most recent call last):
 SyntaxError: cannot assign to conditional expression
 
+>>> a, b += 1, 2
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+
+>>> (a, b) += 1, 2
+Traceback (most recent call last):
+SyntaxError: cannot assign to tuple
+
+>>> [a, b] += 1, 2
+Traceback (most recent call last):
+SyntaxError: cannot assign to list
+
 From compiler_complex_args():
 
 >>> def f(None=1):