Fix HTMLParser.handle_startendtag() attrs annotation (#3366)

The value can be None here as well as in handle_starttag().

stdlib/3/html/parser.pyi

@@ -13,7 +13,7 @@ class HTMLParser(ParserBase):
                         attrs: List[Tuple[str, Optional[str]]]) -> None: ...
     def handle_endtag(self, tag: str) -> None: ...
     def handle_startendtag(self, tag: str,
-                           attrs: List[Tuple[str, str]]) -> None: ...
+                           attrs: List[Tuple[str, Optional[str]]]) -> None: ...
     def handle_data(self, data: str) -> None: ...
     def handle_entityref(self, name: str) -> None: ...
     def handle_charref(self, name: str) -> None: ...