backported the code to python 3.5 (and may be older) using futurize-fstrings

[wecacuee]

To make fstrings work for python-3.5 and older I used the futurize-fstrings pip package.

[coveralls]

Coverage increased (+0.009%) to 99.468% when pulling 911e6ee on wecacuee:try-futurize-fstring into 3a356c6 on bofm:master.

[bofm]

First of all thanks for the contribution.

I don't accept this PR because:

1. Not supporting Python older than 3.6 is actually a feature.
2. I don't see any reason to backport any Python library from 3.6 to 3.5 instead of upgrading your project to 3.6 or 3.7.
3. I personally don't like the approach with future-fstrings.

I'll keep the PR open. Maybe someone is interested.