Use tox and Github actions in all the supported add-ons.

[JakaKokosar]

For the sake of consistency migrate from Travis to Github actions and start using tox across all the supported add-ons.

List of supported add-ons (https://github.com/biolab/orange3-addons):

• [X] Orange3-Associate (https://github.com/biolab/orange3-associate/pull/37)
• [x] Orange3-Bioinformatics (https://github.com/biolab/orange3-bioinformatics/pull/280)
• [x] Orange3-Educational (https://github.com/biolab/orange3-educational/pull/100)
• [X] Orange3-Explain (https://github.com/biolab/orange3-explain/pull/6)
• [x] Orange3-Geo (https://github.com/biolab/orange3-geo/pull/120)
• [x] Orange3-ImageAnalytics (https://github.com/biolab/orange3-imageanalytics/pull/190)
• [x] Orange3-Network (https://github.com/biolab/orange3-network/pull/177)
• [x] Orange3-Prototypes (https://github.com/biolab/orange3-prototypes/pull/231)
• [x] Orange3-SingleCell (https://github.com/biolab/orange3-single-cell/pull/381)
• [x] Orange-Spectroscopy (https://github.com/Quasars/orange-spectroscopy/pull/507)
• [x] Orange3-Text (https://github.com/biolab/orange3-text/pull/613)
• [x] Orange3-Timeseries (https://github.com/biolab/orange3-timeseries/pull/145)

Unofficially official:

• [x] Orange3-Survival-Analysis (https://github.com/biolab/orange3-survival-analysis/pull/6)

We want add-ons to test against different versions of Orange, orange-canvas and widget-base. As discussed with @markotoplak, we proposed the following tox environments:

• orange-oldest will test against least supported (roughly) versions
• orange-latest will test against the current master branch
• orange-released will test against currently released versions

Add-ons will be tested on all three major platforms and Python versions 3.7 and 3.8. It will be possible to easily start experimental testing with python 3.9 when we decide to.

[JakaKokosar]

Closing this issue, migration is complete.