HTMLParser: Add missing stubs for attributes and methods (#6658)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

stdlib/html/parser.pyi

@@ -1,5 +1,5 @@
 from _markupbase import ParserBase
-from typing import Tuple
+from typing import Pattern, Tuple
 
 class HTMLParser(ParserBase):
     def __init__(self, *, convert_charrefs: bool = ...) -> None: ...
@@ -28,3 +28,8 @@ class HTMLParser(ParserBase):
     def parse_pi(self, i: int) -> int: ...  # undocumented
     def parse_starttag(self, i: int) -> int: ...  # undocumented
     def set_cdata_mode(self, elem: str) -> None: ...  # undocumented
+    rawdata: str  # undocumented
+    cdata_elem: str | None  # undocumented
+    convert_charrefs: bool  # undocumented
+    interesting: Pattern[str]  # undocumented
+    lasttag: str  # undocumented