bpo-24612: Improve syntax error for 'not' after an operator (GH-28170)

Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>

Lib/test/test_syntax.py

@@ -1712,6 +1712,49 @@ SyntaxError: only single target (not tuple) can be annotated
 Traceback (most recent call last):
 SyntaxError: only single target (not list) can be annotated
 
+# 'not' after operators:
+
+>>> 3 + not 3
+Traceback (most recent call last):
+SyntaxError: 'not' after an operator must be parenthesized
+
+>>> 3 * not 3
+Traceback (most recent call last):
+SyntaxError: 'not' after an operator must be parenthesized
+
+>>> + not 3
+Traceback (most recent call last):
+SyntaxError: 'not' after an operator must be parenthesized
+
+>>> - not 3
+Traceback (most recent call last):
+SyntaxError: 'not' after an operator must be parenthesized
+
+>>> ~ not 3
+Traceback (most recent call last):
+SyntaxError: 'not' after an operator must be parenthesized
+
+>>> 3 + - not 3
+Traceback (most recent call last):
+SyntaxError: 'not' after an operator must be parenthesized
+
+>>> 3 + not -1
+Traceback (most recent call last):
+SyntaxError: 'not' after an operator must be parenthesized
+
+# Check that we don't introduce misleading errors
+>>> not 1 */ 2
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+
+>>> not 1 +
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+
+>>> not + 1 +
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+
 Corner-cases that used to fail to raise the correct error:
 
     >>> def f(*, x=lambda __debug__:0): pass