Adding checks if music21 is installed

Music21 is optional dependency

ziffers/converters.py

@@ -1,10 +1,18 @@
 """Collection of converters"""
-from music21 import converter, note, stream, meter, chord, environment
 from ziffers import zparse, Ziffers, Pitch, Rest, Chord
 
+try:
+    from music21 import converter, note, stream, meter, chord, environment
+    music21_imported: bool = True
+except ImportError:
+    music21_imported: bool = False
 
 def to_music21(expression: str | Ziffers, **options):
     """Helper for passing options to the parser"""
+
+    if not music21_imported:
+        raise ImportError("Install Music21 library")
+
     converter.registerSubconverter(ZiffersMusic21)
 
     if isinstance(expression, Ziffers):
@@ -32,6 +40,9 @@ def set_musescore_path(path: str):
     settings["musescoreDirectPNGPath"] = path
 
 
+if music21_imported:
+
+    # pylint: disable=locally-disabled, invalid-name, unused-argument, attribute-defined-outside-init
     class ZiffersMusic21(converter.subConverters.SubConverter):
         """Ziffers converter to Music21"""
 
@@ -68,4 +79,5 @@ class ZiffersMusic21(converter.subConverters.SubConverter):
                     m_item = chord.Chord(item.notes)
                     m_item.duration.quarterLength = item.duration * 4
                 note_stream.append(m_item)
+            # TODO: Is this ok?
             self.stream = note_stream.makeMeasures()