Typically there are more packages than what's in the dist-packages, or even what's in the python2.7 directory. In order to do what you're trying to achieve this way you'd need to find all of those locations and add them to your sys.path .
What might be a better approach is to look at using the PyDen app: https://splunkbase.splunk.com/app/4322/. This app (and it's companion PyDen Manager) will allow you to create Python virtual environments of any version you can compile on your system (including version 3.5+) that can be used inside of Splunk. The app comes with a pip tool that will allow you to install and PyPI package to a specific virtual environment and there are examples scripts of how to activate the virtual environments from your Python scripts.
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