Modernize the code to Python 3.6+

[eumiro]

Now that we're at Python 3.6+, we can modernize the code and add f-strings, pathlib.Path and other Python3 features.

Next steps would be to convert the tests to pytest. What do you think?

[coveralls]

Coverage decreased (-1.0%) to 99.035% when pulling 69f75d0 on eumiro:py36plus into d2c9f71 on jsvine:master.

[jsvine]

Thanks! On the whole, I like the ideas here. But this PR introduces a lot of line-changes, spanning a few different suggestions, so it'll take me a little while to audit it. (I realize that many of the changes are auto-generated, but I'd still like to read them carefully.)

[eumiro]

Sorry for that huge diff. Would you like me to split different topics better into individual commits? That's a big change, but while the best moment to set formatting standards was at the beginning of the project, the second best is now.

[jsvine]

I think it's fine to keep this PR as-is, but thanks for offering. I haven't forgotten about the PR, but have been busy. I hope to take a closer look soon.

[jsvine]

Thanks again, @eumiro. I've taken some of these ideas, and some of the commits directly, and incorporated them into #166, now merged.

One thing I didn't do was to use pathlib; it's a good idea, but does not play a major role here. If you'd like, feel free to submit a PR specifically for that.