Well, that depends on what you need to support; even if RHEL7 is the oldest distribution that you need to support, you can still assume you have at least Python

Well, that depends on what you need to support; even if RHEL7 is the oldest distribution that you need to support, you can still assume you have at least Python 3.6, or python 3.4 for RHEL6 (which is already EOL). There's no obligation to support every potential platform your script might ever need to run on until that need actually arises. You just need to define what platforms you want to support and work within their constraints. Unfortunately for lots of software that platform is essentially "whatever the developer managed to install on their system". Constraining yourself to support a particular platform might mean there will be libraries you will not be able to use even if they might be useful, but that's the tradeoff you make when building software against a stable platform.