Cleanup python 2 leftovers

[tritemio]

Overview

FRETBursts 0.7+ drops support for python 2.7. It is time to remove some old cruft!

This is an easy issue that can help first-time contributors to get the hands dirty with FRETBursts codebase.

Technical description

FRETBursts uses the future package to assure python 2 compatibility:

https://python-future.org/compatible_idioms.html

Imports "from future" can be removed now. In particular, these lines be safely removed

• from __future__ import ...
• from builtins import ...
• check other imports of past, future and builtins.

Also the future package should be removed from the dependencies (see setup.py).

If you use FRETBursts this is a good issue to start contributing!