Smart indent (shift+tab) for python code results in bad indentation

When selecting a piece of python script and pressing SHIFT+TAB, the indentation of the selected code will not respect its initial goal (i.e. will not be preserved relative to its inial state).

Caused by a bug in upstream CodeMirror's python.js mode, still open even in the current 5.33 version.

Until the upstream project or we manage to find a solution, the workaround would be to simply not use the SHIFT+TAB feature and manually handle deindentation, where needed.