bpo-43797: Improve syntax error for invalid comparisons (#25317)

* bpo-43797: Improve syntax error for invalid comparisons

* Update Lib/test/test_fstring.py

Co-authored-by: Guido van Rossum <gvanrossum@gmail.com>

* Apply review comments

* can't -> cannot

Co-authored-by: Guido van Rossum <gvanrossum@gmail.com>
diff --git a/Lib/test/test_genexps.py b/Lib/test/test_genexps.py
index 5c1a209..70fe2bb 100644
--- a/Lib/test/test_genexps.py
+++ b/Lib/test/test_genexps.py
@@ -103,7 +103,7 @@
     >>> dict(a = i for i in range(10))
     Traceback (most recent call last):
        ...
-    SyntaxError: invalid syntax
+    SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?
 
 Verify that parenthesis are required when used as a keyword argument value