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

# 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.