Improve stubs for email.policy (#6631)

* Add the ABCMeta metaclass, present at runtime.
* Add missing attributes from the allowlist, which are documented.
* Annotate the __init__ methods according to the documentation.

stdlib/email/policy.pyi

@@ -1,17 +1,27 @@
-from abc import abstractmethod
+from abc import ABCMeta, abstractmethod
 from email.contentmanager import ContentManager
 from email.errors import MessageDefect
 from email.header import Header
 from email.message import Message
 from typing import Any, Callable
 
-class Policy:
+class Policy(metaclass=ABCMeta):
     max_line_length: int | None
     linesep: str
     cte_type: str
     raise_on_defect: bool
-    mange_from: bool
-    def __init__(self, **kw: Any) -> None: ...
+    mangle_from_: bool
+    message_factory: Callable[[Policy], Message] | None
+    def __init__(
+        self,
+        *,
+        max_line_length: int | None = ...,
+        linesep: str = ...,
+        cte_type: str = ...,
+        raise_on_defect: bool = ...,
+        mangle_from_: bool = ...,
+        message_factory: Callable[[Policy], Message] | None = ...,
+    ) -> None: ...
     def clone(self, **kw: Any) -> Policy: ...
     def handle_defect(self, obj: Message, defect: MessageDefect) -> None: ...
     def register_defect(self, obj: Message, defect: MessageDefect) -> None: ...
@@ -41,6 +51,20 @@ class EmailPolicy(Policy):
     refold_source: str
     header_factory: Callable[[str, str], str]
     content_manager: ContentManager
+    def __init__(
+        self,
+        *,
+        max_line_length: int | None = ...,
+        linesep: str = ...,
+        cte_type: str = ...,
+        raise_on_defect: bool = ...,
+        mangle_from_: bool = ...,
+        message_factory: Callable[[Policy], Message] | None = ...,
+        utf8: bool = ...,
+        refold_source: str = ...,
+        header_factory: Callable[[str, str], str] = ...,
+        content_manager: ContentManager = ...,
+    ) -> None: ...
     def header_source_parse(self, sourcelines: list[str]) -> tuple[str, str]: ...
     def header_store_parse(self, name: str, value: str) -> tuple[str, str]: ...
     def header_fetch_parse(self, name: str, value: str) -> str: ...