Splunk v 8 has Python 3 onboard. I do not believe that there are plans to upgrade Python on the lower versions of the application.
The documentation bits on Python 3 are found in : https://docs.splunk.com/Documentation/Splunk/8.0.2/Python3Migration/AboutMigration
If you are already on splunk 8 and you want to restrict to Python 3 only, put the following in $SPLUNK_HOME/etc/system/local/server.conf
[general]
python.version = python3
And restart the Splunk system.
