Migrate to Python 3

[vterron]

Python 2 reached its EOL on January 1st, 2020.

[...] no new bug reports, fixes, or changes will be made to Python 2, and Python 2 is no longer supported.

https://www.python.org/doc/sunset-python-2/

[stale]

Thanks for contributing to this issue. As it has been 180 days since the last activity, we'll be automatically closing the issue soon. This is often because the request was already solved in some way and it just wasn't updated or it's just no longer relevant. If that's not the case, please respond here within the next 30 days to keep it open. You can read more here: https://github.com/probot/stale#is-closing-stale-issues-really-a-good-idea.

[vterron]

Keep.