Change test of assignment of backticked expression to be an "invalid syntax"

SyntaxError.

This is probably not the proper solution to this failing test, but removing the
test itself causes 19 other tests to fail for some odd reason because doctest
doesn't expect a complete traceback (or something; rather odd problem for just
removing a single test).

Lib/test/test_syntax.py

@@ -71,7 +71,7 @@ SyntaxError: can't assign to literal (<doctest test.test_syntax[9]>, line 1)
 
 >>> `1` = 1
 Traceback (most recent call last):
-SyntaxError: can't assign to repr (<doctest test.test_syntax[10]>, line 1)
+SyntaxError: invalid syntax
 
 If the left-hand side of an assignment is a list or tuple, an illegal
 expression inside that contain should still cause a syntax error.