Avoid UserWarning when creating ClientSession outside a coroutine

If an aiohttp.ClientSession is created outside a coroutine without a running
event loop, a warning is shown:
/Users/walkman/Nextcloud/Projects/simple-podcast-dl/podcast_dl/cli.py:210: UserWarning: Creating a client session outside of coroutine is a very dangerous idea
  http = aiohttp.ClientSession()
Creating a client session outside of coroutine
client_session: <aiohttp.client.ClientSession object at 0x102cb5ef0>

We can avoid it by explicitly passing an event loop. For details, see:
aio-libs/aiohttp#1468

podcast_dl/cli.py

@@ -207,12 +207,12 @@ def main(
 
     vprint = click.secho if verbose else noprint
 
-    http = aiohttp.ClientSession()
-    atexit.register(http.close)
-
     loop = asyncio.get_event_loop()
     atexit.register(loop.close)
 
+    http = aiohttp.ClientSession(loop=loop)
+    atexit.register(http.close)
+
     ensure_download_dir(download_dir)
     rss_root = loop.run_until_complete(download_rss(http, podcast.rss))
     all_rss_items = get_all_rss_items(rss_root, podcast.rss_parser)