Add support for Python 3.10dev

[humitos]

When upgrading to Ubuntu 20.04 it would be good to include Python 3.10dev together with Python 3.9 so we allow start testing Python 3.10 before it gets released. Once it's released, we can just update to the 3.10.0 version.

[pradyunsg]

Ahoy! Anything that can be done to help move this forward?

[humitos]

I opened #174 and we hope to review, merge and deploy soon 😃

[humitos]

This is already done. You can use the build.image: testing image and python.version: "3.10" now in the YAML config file.

Please, let us know if it works for you!