Add all the email __all__s (#7304)

stdlib/email/parser.pyi

@@ -3,6 +3,8 @@ from email.message import Message
 from email.policy import Policy
 from typing import BinaryIO, Callable, TextIO, TypeVar
 
+__all__ = ["Parser", "HeaderParser", "BytesParser", "BytesHeaderParser", "FeedParser", "BytesFeedParser"]
+
 _M = TypeVar("_M", bound=Message)
 
 FeedParser = email.feedparser.FeedParser[_M]