Add support for .m3u playlists (#401)

* Add support for .m3u playlists

* Run black code formatter on changes

* Stay consistent with Spotify test track

test/test_handle.py

@@ -3,12 +3,22 @@ import sys
 import argparse
 
 from spotdl import handle
-from spotdl import const
 
 import pytest
 import yaml
 
 
+def test_error_m3u_without_list():
+    with pytest.raises(SystemExit):
+        handle.get_arguments(raw_args=('-s cool song', '--write-m3u',),
+                             to_group=True)
+
+
+def test_m3u_with_list():
+    handle.get_arguments(raw_args=('-l cool_list.txt', '--write-m3u',),
+                         to_group=True)
+
+
 def test_log_str_to_int():
     expect_levels = [20, 30, 40, 10]
     levels = [handle.log_leveller(level) for level in handle._LOG_LEVELS_STR]