bpo-44273: Improve syntax error message for assigning to "..." (GH-26477)

Use "ellipsis" instead of "Ellipsis" in syntax error messages to eliminate confusion with built-in variable Ellipsis.

Lib/test/test_syntax.py

@@ -93,7 +93,7 @@ SyntaxError: cannot assign to literal here. Maybe you meant '==' instead of '='?
 
 >>> ... = 1
 Traceback (most recent call last):
-SyntaxError: cannot assign to Ellipsis here. Maybe you meant '==' instead of '='?
+SyntaxError: cannot assign to ellipsis here. Maybe you meant '==' instead of '='?
 
 >>> `1` = 1
 Traceback (most recent call last):

Parser/pegen.c

@@ -217,7 +217,7 @@ _PyPegen_get_expr_name(expr_ty e)
                 return "True";
             }
             if (value == Py_Ellipsis) {
-                return "Ellipsis";
+                return "ellipsis";
             }
             return "literal";
         }