Fix parsing "none" for --slice

Commit 8d72c4d introduced a regression where the `--slice` CLI argument or other calls to the None parser with a custom set of allowed "None" values could be misinterpreted, leading to an error.

magmap/io/cli.py

@@ -81,7 +81,7 @@ def _parse_none(
 
     """
     if not libmag.is_seq(none):
-        none = tuple(none)
+        none = [none]
     if arg.lower() in none:
         return None
     return arg if fn is None else fn(arg)