lcitool: add support for newer Python toolchains on CentOS 8 and OpenSUSE LEAP 15

In some cases, the same OS might provide multiple versions of a package (Python being the case in point), but with different functionality. For example, both CentOS Stream 8 and OpenSUSE LEAP 15.3 provide a "platform" Python, which is version 3.6 as of this writing, with a full runtime; and newer Python versions for which only the basic runtime is provided.

Such newer runtimes are generally limited from the point of view of lcitool, and therefore it is only desirable to use them if some of the packages in the build do not support Python 3.6---which is currently at end of life.

Therefore, introduce a new way to differentiate targets beyond OS "name" and "version", the variant. Variants can change the mapping of packages such as the Python interpreter, in order to bring in the required version of the runtime. The merge request introduces two such targets, CentOS Stream 8 + Python 3.8 and OpenSUSE LEAP 15.3 + Python 3.9 respectively.