popen fails on Raspberry Pi

subprocess.Popen() encoding parameter was added in Python 3.6 and will cause the call to fail on earlier versions of Python. Perhaps don't bother with the parameter or, if it is really necessary, try the command with it and then try without if it fails?

I am running Thonny on Raspbian OS and Ubuntu and the default Python3 installation (via apt-get) is only 3.5.3 currently.