IMHO it’s solved by installing different python versions using the OS package manager, like `dnf install python3.8 python3.9`, and then creating virtualenv using whichever version you need, like `virtualenv -p python3.9 venv`
You'll be limited to whatever python version is packaged on the host operating system. pyenv can let you test against development versions of python if needed, or released of python that are either new enough (or old enough in some cases) that they aren't available on your operating system's package repositories.
You can `make dist` on the python repo to build the rpm of whichever branch/commit you want to use. Pre-built ones are also available from nightly build. I still don’t know why you need a separate tool for that.