Annotate HTMLParser attribute optional value (#3034)

Closes #3031

stdlib/3/html/parser.pyi

@@ -1,4 +1,4 @@
-from typing import List, Tuple
+from typing import List, Optional, Tuple
 from _markupbase import ParserBase
 import sys
 
@@ -15,7 +15,7 @@ class HTMLParser(ParserBase):
     def get_starttag_text(self) -> str: ...
 
     def handle_starttag(self, tag: str,
-                        attrs: List[Tuple[str, str]]) -> None: ...
+                        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: ...