Ensure compatibility with both sync and async contents managers

[yjoer]

Following #61, I inspected the class used by the instance bounded to self.contents_manager by printing its type before the should_hide(...) function returns. This is what I got before installing Jupytext.

<class 'jupyter_server.services.contents.largefilemanager.AsyncLargeFileManager'>

After installing Jupytext, the contents manager changed.

<class 'jupytext.contentsmanager.build_jupytext_contents_manager_class.<locals>.JupytextContentsManager'>

After digging for a while, Jupytext does not support an async contents manager and overrides the contents manager with a subclass of LargeFileManager since 1.14.3.

Along these lines, I came up with a change to ensure we can call both sync and async methods.

[github-actions]

👈 Launch a Binder on branch yjoer/jupyterlab-quickopen/fix-1

[jtpio]

Thanks @yjoer for working on this!

Feel free to undraft the PR if you feel like this is ready for review and merge.

[yjoer]

I tested this manually by uninstalling and reinstalling Jupytext. It should be good to go.

[jtpio]

Thanks!

[jtpio]

Released in https://github.com/jupyterlab-contrib/jupyterlab-quickopen/releases/tag/v2.0.1