Switch from Travis CI to GitHub workflows

[sbesson]

• add tox.ini configuration to run tests in multiple environments
• set-up Tox and PyPI publication workflows
• fix deprecated Pytest fixtures
• remove Travis CI

See https://github.com/sbesson/omero-marshal/actions/runs/391224454 and https://github.com/sbesson/omero-marshal/actions/runs/391224455

[jburel]

20.04 is going to be the default shortly (ubuntu-latest). Any particular reason to pin to 18.04?

[sbesson]

This is mostly because we have precompiled Ice wheels for 3.6/3.7/3.8 for this environement used in tox.ini. Happy to switch if we have equivalent artifacts for Ubuntu 20.04 and the three Python versions

[joshmoore]

3.9 should also exist now ;)

[sbesson]

I am fine with Python 3.6/3.7/3.8/3.9 on Ubuntu 20.04 too. Should all the wheels be generated as release artifacts of https://github.com/ome/zeroc-ice-ubuntu2004 then?

[manics]

miniconda is installed by default, we could skip these custom built wheels and use that instead?

[sbesson]

For reference, https://github.com/sbesson/omero-marshal/actions/runs/393725465 is the result of the execution on Ubuntu 20.04, with Python 3.9 and without the custom wheels. The run time increased from ~1min to ~10min largely because of the installation of zeroc-ice but if we all agree this is acceptable, I can push the change to this PR

[sbesson]

Looking at https://github.com/ome/omero-py/blob/v5.8.3/tox.ini and https://github.com/ome/omero-py/blob/v5.8.3/.github/workflows/workflow.yml, I realized that omero-py is actually running on Ubuntu 20.04 and using the wheels from https://github.com/ome/zeroc-ice-py-travis-bionic.

f3e48e3 re-enables these wheels for Python 3.6, 3.7 and 3.8 - see https://github.com/sbesson/omero-marshal/actions/runs/395673955

I could however use some clarification on:

• whether we will keep supporting these wheels moving forward
• if yes, whether we will build them for all supported Python versions - Actions zeroc-ice-ubuntu2004#1 for instance only creates one for the default Python on the platform

[sbesson]

Last commit consumes the wheels now built by https://github.com/ome/zeroc-ice-py-github-ci - see https://github.com/sbesson/omero-marshal/actions/runs/423553340 for the execution

[jburel]

Tested on my fork.
Everything works as expected