What happens if you right-click on the python file and choose “open-with → notebook”? With my install that opens the file as a notebook in a new tab. Once the file is open, the pairing options are avalilable in the command palette as described in Installation — Jupytext documentation. There’s work in progress to improve this here: Enhance Jupytext for JupyterLab by fcollonval · Pull Request #877 · mwouts/jupytext · GitHub
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