Background: https://www.jeffgeerling.com/blog/2020/travis-cis-new-pricing-plan-threw-wrench-my-open-source-works

It's not possible to use Travis to build the Linux and macOS wheels due to changes in Travis pricing.

To do:

• Select a new CI provider
• Check if it is possible to use multibuild with the new CI provider
• Migrate the Travis side of the build toolchain to the new CI provider

Related issues:

#421