Documentation: mention correct python versions

[penguinolog]

• Readme
• Classifiers

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

[coveralls]

coverage: 73.507% (+0.004%) from 73.503% when pulling 7391b2c on penguinolog:py_version_description into cce5326 on urwid:master.