Update GitHub Actions workflow Ubuntu image from 20.04 to latest #112
Description
Summary
The 20.04 Ubuntu image is no longer available in GitHub Actions. We need to replace it with 22.04 or latest.
The reason we have intentionally kept it at 20.04 is because that was the only Ubuntu image where Python 3.6 was available for the test matrix. But maybe it is time to ditch version of Python that no longer receive security patches. That means all Python versions up to 3.9, which is still supported.
If we do that, then we can safely update to ubuntu-latest in the workflow.