Tests fail if `notebook_shim` is installed

[krassowski]

This is more to help anyone who gets this error to resolve it than an actual bug in this repo - if you see:

    def sorted_extensions(self):
        """Dictionary with extension package names as keys
        and an ExtensionPackage objects as values.
        """
        # Sort the keys and
        keys = sorted(self.extensions.keys())
>       keys.remove("notebook_shim")
E       ValueError: list.remove(x): x not in list

site-packages/notebook_shim/nbserver.py:90: ValueError

when running tests, uninstall notebook_shim to get the tests passing.