Changes due to upstream libs

[kafonek]

• fix https://github.com/kafonek/ipython_blocking/issues/15, use the new run_sync decorator instead of old nbclient.utils.just_run
• Move code into src/ and use poetry to manage deps instead of setup.py

New (?) or unresolved issues:

• %blockrun isn't working, seems like Jupyter.notebook.select_next().execute_cells_below() is no longer valid
• The cell that has the context manager or %block magic is showing its state as executing even once it's broken out of and other cells have replayed. New cells can be executed, but the incorrect status still shows up in frontend