Update to JupyterLab 3.0 RC

[ajbozarth]

Update Elyra to run on JupyterLab 3.0 RC

The following temporary changes will need to be reverted prior to the next release:

• adding --pre to applicable pip installs
• commenting out dependancies and installations of the git extension

Fixes #832

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[ajbozarth]

As I was opening this I noticed flake8 was commented out in the Makefile. I'm currently checking if that needs to be addressed and will push an update if it works as is, otherwise fixing flake will be a followup issue

[jtpio]

Just curious: do you also plan to adopt the new distribution system in lab 3.0 to distribute each extension separately as Python packages?

Or bundle them all in the main elyra package similar to the current approach?

elyra/setup.py

Line 102 in 19a484f

setup_args["data_files"].append(('share/jupyter/lab/extensions', glob(npm_packages_path)))

[ajbozarth]

do you also plan to adopt the new distribution system in lab 3.0

We plan to initially continue using npm for our extension packages, as seen in this PR, to release 3.0 support asap. Then in a future release we plan to update to the new federated extensions, we have not decided how soon that update will happen though.