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/policy.pyi

@@ -1,14 +1,12 @@
 from collections.abc import Callable
-from email._policybase import Compat32 as Compat32, Policy as Policy, _MessageFactory, compat32 as compat32
+from email._policybase import Compat32 as Compat32, Policy as Policy, _MessageFactory, _MessageT, compat32 as compat32
 from email.contentmanager import ContentManager
-from email.message import EmailMessage, Message
-from typing import Any, TypeVar, overload
+from email.message import EmailMessage
+from typing import Any, overload
 from typing_extensions import Self
 
 __all__ = ["Compat32", "compat32", "Policy", "EmailPolicy", "default", "strict", "SMTP", "HTTP"]
 
-_MessageT = TypeVar("_MessageT", bound=Message, default=Message)
-
 class EmailPolicy(Policy[_MessageT]):
     utf8: bool
     refold_source: str