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/mime/message.pyi

@@ -1,5 +1,6 @@
+from email._policybase import _MessageT
 from email.mime.nonmultipart import MIMENonMultipart
-from email.policy import Policy, _MessageT
+from email.policy import Policy
 
 __all__ = ["MIMEMessage"]
 

stdlib/email/mime/multipart.pyi

@@ -1,7 +1,8 @@
 from collections.abc import Sequence
 from email import _ParamsType
+from email._policybase import _MessageT
 from email.mime.base import MIMEBase
-from email.policy import Policy, _MessageT
+from email.policy import Policy
 
 __all__ = ["MIMEMultipart"]