app.py

@bot.command(name='join', help='Tells the bot to join the voice channel')
async def join(ctx):
    if not ctx.message.author.voice:
        await ctx.send("{} is not connected to a voice channel".format(ctx.message.author.name))
        return
    else:
        channel = ctx.message.author.voice.channel
    await channel.connect()

@bot.command(name='leave', help='To make the bot leave the voice channel')
async def leave(ctx):
    voice_client = ctx.message.guild.voice_client
    if voice_client.is_connected():
        await voice_client.disconnect()
    else:
        await ctx.send("The bot is not connected to a voice channel.")