Migrate CI to GitHub actions

[sbesson]

Largely inspired by the work in omero-py this migrates the CI to using GitHub actions and also adds coverage for Python 3.7 and 3.8

• create GitHub workflow with 3 steps: install from source, test and publish
• remove Travis-specific environment from tox.ini and use markers like omero-py
• expand envlist to run tests under Python 3.7 and 3.8
• remove .travis.yml
• update CI badge and add badge for PyPI

[joshmoore]

In general, big 👍. I'd still like to see some for of re-use, but I'm failing to see an easy path forward where changes can be propagated:

• https://docs.github.com/en/free-pro-team@latest/actions/learn-github-actions/sharing-workflows-with-your-organization#creating-a-workflow-template -- allows copying for the first instance, but updates don't get propagated
• https://github.community/t/reusing-sharing-inheriting-steps-between-jobs-declarations/16851/17 focuses on steps between jobs rather than jobs between workflows

[manics]

To reuse this you'd probably want to split it into several github actions which do one thing, e.g. install standard dependencies, run standard scripts, a bit like https://github.com/ome/omero-test-infra but with one action per step (and with less coupling?) instead of bundling everything together.

[sbesson]

Folllowing the preliminary work on omero-cli-zarr and ome/.github#4, I went ahead and split the single workflow into 3 workflows. The PyPI publication should match the proposed template and I would assume the Tox one could also be turned into a template

[sbesson]

Relisting for a last round of comments and objections to merging this in the current state?
Proposed next step as a few of our Python libraries are using tox is to test the tox workflow and eventually convert it into a template.

[joshmoore]

In general, big 👍. Some of the badges are hard to confirm until after this is merged, but certainly no objections to iterating through this. Thanks.