Adopt a uniform CI testing solution

[bobfox]

Adopt a uniform CI testing solution that accommodates all three of the major OS platforms (Windows, Linux, macOS).

[altendky]

As I recall, Travis and AppVeyor both list support for all three platforms. In my experience (it's been awhile now, admittedly) Travis did not support more than one Python 3 version on either Windows or macOS. I don't know where AppVeyor stands on that front. I do have multiple projects building on Windows/Linux/macOS and Python 2.7/3.5-3.8 in Azure Pipelines. I'll note that they did drop the compiler used for any dependencies that need compilation in Python 2.7 though IIRC it's still installable via chocolatey. Also, 2.7 is EOL so... but sure, some libraries are continuing support.
From what I hear GitHub Actions is based on the same underlying system as Azure Pipelines with some top end differences so presumably it can also cover the matrix well.