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/parser.pyi

@@ -1,20 +1,21 @@
 from _typeshed import SupportsRead
 from collections.abc import Callable
+from email._policybase import _MessageT
 from email.feedparser import BytesFeedParser as BytesFeedParser, FeedParser as FeedParser
 from email.message import Message
 from email.policy import Policy
 from io import _WrappedBuffer
-from typing import Generic, TypeVar, overload
+from typing import Generic, overload
 
 __all__ = ["Parser", "HeaderParser", "BytesParser", "BytesHeaderParser", "FeedParser", "BytesFeedParser"]
 
-_MessageT = TypeVar("_MessageT", bound=Message, default=Message)
-
 class Parser(Generic[_MessageT]):
     @overload
-    def __init__(self: Parser[Message[str, str]], _class: None = None, *, policy: Policy[Message[str, str]] = ...) -> None: ...
+    def __init__(self: Parser[Message[str, str]], _class: None = None) -> None: ...
     @overload
-    def __init__(self, _class: Callable[[], _MessageT], *, policy: Policy[_MessageT] = ...) -> None: ...
+    def __init__(self, _class: None = None, *, policy: Policy[_MessageT]) -> None: ...
+    @overload
+    def __init__(self, _class: Callable[[], _MessageT] | None, *, policy: Policy[_MessageT] = ...) -> None: ...
     def parse(self, fp: SupportsRead[str], headersonly: bool = False) -> _MessageT: ...
     def parsestr(self, text: str, headersonly: bool = False) -> _MessageT: ...
 
@@ -25,9 +26,9 @@ class HeaderParser(Parser[_MessageT]):
 class BytesParser(Generic[_MessageT]):
     parser: Parser[_MessageT]
     @overload
-    def __init__(
-        self: BytesParser[Message[str, str]], _class: None = None, *, policy: Policy[Message[str, str]] = ...
-    ) -> None: ...
+    def __init__(self: BytesParser[Message[str, str]], _class: None = None) -> None: ...
+    @overload
+    def __init__(self, _class: None = None, *, policy: Policy[_MessageT]) -> None: ...
     @overload
     def __init__(self, _class: Callable[[], _MessageT], *, policy: Policy[_MessageT] = ...) -> None: ...
     def parse(self, fp: _WrappedBuffer, headersonly: bool = False) -> _MessageT: ...