Move to GitHub Actions for testing

The current legacy QA is missing several Python versions (3.5, 3.7, 3.8, 3.9) thus that cannot be tested. There is also the fact that we cannot have PR status as the QA is private. There is a job, but it has to be run manually.

So just move to GitHub Actions using Nuxeo from a docker image and use that test matrix:

• Python 2.7 (NXPY-167)
• Python 3.5 (NXPY-99)
• Python 3.6
• Python 3.7
• Python 3.8
• (Python 3.9)

And:

• Nuxeo 9.10
• Nuxeo 10.10
• Nuxeo master

Just use Linux as the VM and start the docker image from a service.

The workflow should be called eveytime a push is done on a PR.

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Old Jenkins jobs will be disabled: python-client-dynamic and nuxeo-python-client-master.