extend python feature with "system, but at least 3.x"

[AlexanderLanin]

Background

Currently we can specify a very specific version or the system version. Since the feature is supposed to run on a bunch of different base images, specifying system is not helpful except in tightly controlled environments. Specifying a specific version like 3.8 could be overkill, in case the system already provides e.g. 3.8.1 precompiled.

Feature Request

It would be nice to say "use the system version, but no lower then 3.x". Maybe the python typical notation can be reused: >=3.x to indicate this.