Fix HTMLParser.HTMLParser, HTMLParseError. (#565)

Signed-off-by: Rebecca Chen <rechen@rechen0.mtv.corp.google.com>

stdlib/2.7/HTMLParser.pyi

@@ -2,7 +2,7 @@ from typing import List, Tuple, AnyStr
 from markupbase import ParserBase
 
 class HTMLParser(ParserBase):
-    def __init__(self, *args, convert_charrefs: bool) -> None: ...
+    def __init__(self) -> None: ...
     def feed(self, feed: AnyStr) -> None: ...
     def close(self) -> None: ...
     def reset(self) -> None: ...
@@ -25,4 +25,7 @@ class HTMLParser(ParserBase):
 
     def unescape(self, s: AnyStr) -> AnyStr: ...
 
-class HTMLParseError(Exception): ...
+class HTMLParseError(Exception):
+    msg = ...  # type: str
+    lineno = ...  # type: int
+    offset = ...  # type: int