v2.6.3

This maintainence release contains backports from v2.8.0

Bug Fixes

• Fix Message.author.public_flags always being 0 when the member cache is disabled. ( #870 )
• Export missing ThreadWithMessage class. ( #879 )
• Fix Client.delete_guild_command() not updating the local command cache. ( #907 )
• Fix errors when trying to deserialize stickers with unknown formats. ( #911 )