Forum channels support (#1249)

* Initial forum channel commit

* Update discord/channel.py

Co-authored-by: plun1331 <[email protected]>

* Update discord/abc.py

Co-authored-by: Lala Sabathil <[email protected]>

* Update discord/channel.py

Co-authored-by: Lala Sabathil <[email protected]>

* Update discord/channel.py

Co-authored-by: Lala Sabathil <[email protected]>

* Update discord/channel.py

Co-authored-by: Lala Sabathil <[email protected]>

* Update discord/channel.py

Co-authored-by: Lala Sabathil <[email protected]>

* Update discord/channel.py

Co-authored-by: Lala Sabathil <[email protected]>

* Update discord/channel.py

Co-authored-by: Lala Sabathil <[email protected]>

* Add more forum channel/thread features

* Update discord/channel.py

* apply linting, fix indentation

Co-authored-by: krittick <[email protected]>
Co-authored-by: plun1331 <[email protected]>
Co-authored-by: Lala Sabathil <[email protected]>
Co-authored-by: plun1331 <[email protected]>

Author: 4 people

discord/abc.py

@@ -86,6 +86,7 @@
     from .client import Client
     from .embeds import Embed
     from .enums import InviteTarget
+    from .flags import ChannelFlags
     from .guild import Guild
     from .member import Member
     from .message import Message, MessageReference, PartialMessage
@@ -305,6 +306,7 @@ class GuildChannel:
     - :class:`~discord.VoiceChannel`
     - :class:`~discord.CategoryChannel`
     - :class:`~discord.StageChannel`
+    - :class:`~discord.ForumChannel`
 
     This ABC must also implement :class:`~discord.abc.Snowflake`.
 
@@ -327,6 +329,7 @@ class GuildChannel:
     type: ChannelType
     position: int
     category_id: Optional[int]
+    flags: ChannelFlags
     _state: ConnectionState
     _overwrites: List[_Overwrites]