Clean up and fix email message types (#13532)

* Unify the `_MessageT` type var in `email._policybase`.
* Use explicit type arguments for `Message` type in `_MessageT` type var.
  In particular, change bound from `Message[str, str]` to `Message[Any, Any]`.
* Change `__init__()` overloads of `Parser` and `BytesParser` to accept
  `Message` objects that are not `Message[str, str]` if `_class` is not also given.

stdlib/email/feedparser.pyi

@@ -1,12 +1,11 @@
 from collections.abc import Callable
+from email._policybase import _MessageT
 from email.message import Message
 from email.policy import Policy
-from typing import Generic, TypeVar, overload
+from typing import Generic, overload
 
 __all__ = ["FeedParser", "BytesFeedParser"]
 
-_MessageT = TypeVar("_MessageT", bound=Message, default=Message)
-
 class FeedParser(Generic[_MessageT]):
     @overload
     def __init__(self: FeedParser[Message], _factory: None = None, *, policy: Policy[Message] = ...) -> None: ...