LaunchAgents runs Python script in version 2
As mentioned in the comments, you can use full paths in the XML, e.g.:
&lt;string&gt;/usr/local/bin/python&lt;/string&gt;

launchd isn't aware of environmental variables, like $PATH, unless explicitly set in the XML, so python would default to /usr/bin/python.
https://serverfault.com/questions/111391/use-an-environment-variable-in-a-launchd-script