Include Tests in Sdist But Not Wheel

[blink1073]

This allows us to test on conda-forge with all of the tests, and use the layout recommended by pytest.

I'll follow this up with some CI cleanup.

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[davidbrochart]

@blink1073 I think this is responsible for this error:

ImportError: Error importing plugin "jupyterlab_server.pytest_plugin": No module named 'jupyterlab_server.pytest_plugin'

[blink1073]

Good point, I'll make a fix

[jtpio]

Looks like this makes the JupyterLab tests fail with the latest jupyterlab_server? For example with the latest commit on master:

https://github.com/jupyterlab/jupyterlab/runs/5583340091?check_suite_focus=true

E   ModuleNotFoundError: No module named 'jupyterlab_server.tests'

[jtpio]

Ah I see there is discussion about this starting from this comment: #243 (comment)

[davidbrochart]

I inlined expected_http_error() in jupyterlab/jupyterlab@404fe2e, as @blink1073 suggested.