HTMLParser.get_starttag_text() can return None (#3072)

Author: narusemotoki

stdlib/3/html/parser.pyi

@@ -12,7 +12,7 @@ class HTMLParser(ParserBase):
     def close(self) -> None: ...
     def reset(self) -> None: ...
     def getpos(self) -> Tuple[int, int]: ...
-    def get_starttag_text(self) -> str: ...
+    def get_starttag_text(self) -> Optional[str]: ...
 
     def handle_starttag(self, tag: str,
                         attrs: List[Tuple[str, Optional[str]]]) -> None: ...