Get rid of the dependency to the `future` package

[amotl]

In the long run, I think it would be a good idea to get rid of any need for the future package completely ¹, as mqttwarn now supports Python 3 only.

Originally posted by @amotl in #584 (comment)

Footnotes

1. https://github.com/jpmens/mqttwarn/blob/871a381f0793b3e086609ea55fb248c6be10743b/mqttwarn/core.py#L5 ↩