Update to newer CI worker image

[oyvindio]

The ubuntu1804 SemaphoreCI machine image used in CI builds for this repo is deprecated. For CI builds to continue working properly, the build configuration should be updated to use a newer image. It looks like the target Python version(s) should be updated to for builds to work with the more recent machine images; more information in the deprecation notes here: https://semaphoreci.com/blog/deprecating-ubuntu-18.04-in-semaphore