Use default system Python if 'system' version is requested

[greatflyingsteve]

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Info on the default version of Python a system uses was added in #716, but when the 'system' version of Python is requested, the package name is still python, hard-coded and non-changeable. In CentOS 9, there isn't even a package available named python. If the 'system' Python is requested, use the newly-added default_system_version information, at least for RedHat-family operating systems, instead of relying on every OS to provide a python package (which traditionally referred specifically to Python 2.x, so few distros seem willing to treat the name python as a safe default name for the intended system Python package).