Fix timezone-naive datetime

bot.py

@@ -146,7 +146,7 @@ class Bot(discord.Client):
             if video.get('live_broadcast') == 'upcoming':
                 scheduled_start = video.get('scheduled_start')
                 if scheduled_start:
-                    remaining = (scheduled_start - datetime.datetime.utcnow()).total_seconds()
+                    remaining = (scheduled_start - datetime.datetime.now(datetime.timezone.utc)).total_seconds()
                     for minutes in COUNTDOWN:
                         if remaining > minutes * 60:
                             Timer.schedule(scheduled_start - datetime.timedelta(minutes=minutes), data)