Set notebook encoding explicitely

Otherwise certain notebooks (e.g. 441c646 in hs23) do not decode on Windows. See https://github.com/jupyter/nbformat/issues/181

commander/gui/notebook.py

@@ -27,7 +27,7 @@ class NotebookView(QWebEngineView):
         if key in self._html_cache:
             return self._html_cache[key]
 
-        with path.open("r") as f:
+        with path.open("r", encoding="utf-8") as f:
             nb = nbformat.read(f, as_version=nbformat.NO_CONVERT)
 
         exporter = nbconvert.HTMLExporter()