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/mailbox.pyi

@@ -4,6 +4,7 @@ import sys
 from _typeshed import StrPath, SupportsNoArgReadline, SupportsRead
 from abc import ABCMeta, abstractmethod
 from collections.abc import Callable, Iterable, Iterator, Mapping, Sequence
+from email._policybase import _MessageT
 from types import GenericAlias, TracebackType
 from typing import IO, Any, AnyStr, Generic, Literal, Protocol, TypeVar, overload
 from typing_extensions import Self, TypeAlias
@@ -29,7 +30,6 @@ __all__ = [
 ]
 
 _T = TypeVar("_T")
-_MessageT = TypeVar("_MessageT", bound=Message)
 
 class _SupportsReadAndReadline(SupportsRead[bytes], SupportsNoArgReadline[bytes], Protocol): ...