refactor: Revert "refactor: quench mypy errors" and update mypy

This mostly reverts commit aa54382.

Instead of adding superfluous assertions, let's update mypy to the latest, which
has better parsers that recognize that `if mime[0]:` already guarantees that
`mime[0] is not None` holds.

.pre-commit-config.yaml

@@ -26,6 +26,6 @@ repos:
         types: [python]
         stages: [commit]
 -   repo: https://github.com/pre-commit/mirrors-mypy
-    rev: 'v0.782'  # Use the sha / tag you want to point at
+    rev: 'v0.941'  # Use the sha / tag you want to point at
     hooks:
     -   id: mypy

readalongs/api.py

@@ -89,7 +89,6 @@ 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())
@@ -134,7 +133,6 @@ 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

readalongs/text/make_package.py

@@ -87,7 +87,6 @@ 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