Special syntax error for elif block after else

Previously, having an elif block after an else block would raise a standard syntax error.

Author: swfarnsworth

Grammar/python.gram

@@ -1396,6 +1396,7 @@ invalid_elif_stmt:
 invalid_else_stmt:
     | a='else' ':' NEWLINE !INDENT {
         RAISE_INDENTATION_ERROR("expected an indented block after 'else' statement on line %d", a->lineno) }
+    | 'else' ':' block 'elif' { RAISE_SYNTAX_ERROR("elif not allowed after else")}
 invalid_while_stmt:
     | 'while' named_expression NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
     | a='while' named_expression ':' NEWLINE !INDENT {