SPEC Tools: CI config for SPEC0 testing #177

The oldest version combination of supported version should be included in the CI matrix. It maybe useful to provide a recipe or a tool to easily and automatically do this, rather than manually track and update versions. E.g. pypi-timemachine/tox-timemachine could be used for this.

But even without the timemachines, it maybe benefitical to provide a matrix template that includes both the dev version testings (for SPEC4) and the oldest supported versions testing for SPEC0.