Create aiohttp session in on_ready to avoid separate loops

Fixes `RuntimeError: Timeout context manager should be used inside a
task` errors when querying arkhamdb

ahtcg_bot.py

@@ -30,8 +30,6 @@ class ArkhamDBUpdater(discord.Client):
         with open(self.channel_list_file) as f:
             self.channel_list = set(json.load(f))
 
-        self.arkhamdb_client = ArkhamDBClient()
-
     async def close(self) -> None:
         await self.arkhamdb_client.close()
         await super().close()
@@ -66,6 +64,8 @@ class ArkhamDBUpdater(discord.Client):
         await tree.sync()
 
     async def on_ready(self) -> None:
+        self.arkhamdb_client = ArkhamDBClient()
+
         logging.info(f"Logged in as {self.user} (ID: {self.user.id})")
         expected_permissions = permissions = discord.Permissions(
             read_messages=True,