fix(typing): Message.topic also holds a DirectMessagesTopic

[Danipulok]

What

Message.topic is annotated Optional["types.ForumTopic"] at message.py:655, and in a
channel direct-messages chat it holds a DirectMessagesTopic. message.py:1870, in the
branch guarded by chat.type == enums.ChatType.DIRECT:

parsed_message.topic = await client.get_direct_messages_topics_by_id(
    chat_id=parsed_message.chat.id,
    topic_ids=parsed_message.direct_messages_topic_id
)

The cache branch a few lines above it does the same. So reading message.topic.icon_color
after a ForumTopic-shaped check passes the type checker and raises at runtime in a direct
chat, and the honest code — checking which of the two it is — is reported as unreachable.

The change

The annotation, and the docstring entry that describes it:

topic: Optional[Union["types.ForumTopic", "types.DirectMessagesTopic"]] = None,

I took the assignment as correct and the annotation as the thing that was behind — the
direct-messages topic is a real, separate type with its own fetch method, so narrowing the
other way would mean deciding that message.py:1870 is the bug. If you would rather it went
that way, say so and I will redo it.