refactor: quench mypy errors

readalongs/api.py

@@ -89,6 +89,7 @@ def align(
             **kwargs
         )
 
+        assert cli.align.callback is not None  # to make mypy happy
         cli.align.callback(**align_args)
 
         return (0, None, logging_stream.getvalue())
@@ -133,10 +134,11 @@ def prepare(
                     ),
                     **kwargs
                 )
+                assert cli.prepare.callback is not None  # to make mypy happy
                 cli.prepare.callback(**prepare_args)
         except OSError as e:
             # e.g.: FileNotFoundError or PermissionError on open(plaintextfile) above
-            raise click.UsageError(e) from e
+            raise click.UsageError(str(e)) from e
 
         return (0, None, logging_stream.getvalue())
     except Exception as e:

readalongs/text/make_package.py

@@ -87,6 +87,7 @@ def encode_from_path(path: str) -> str:
         # TODO: Check other popular audio formats, .wav, .mp3, .ogg, etc...
         mime_type = "audio/mp4"
     elif mime[0]:
+        assert mime[0] is not None  # keep mypy happy. Q why is `if mime[0]` not enough?
         mime_type = mime[0].replace(
             "video", "audio"
         )  # Hack: until we properly extract audio from video files, force any video-based mime type to be read as audio