refactor: reformat Python code using Python 3.6+ syntax

[shuuji3]

Hello, I knew this library through the electricity-map. 🙂

resolves #15

This PR apply pyupgrade with python 3.6+ option so that it can use f-string as well as other simpler expressions.

I run pyupgrade with this command: pipx run pyupgrade --py36-plus *.py.

Also, I updated tox.ini config.