Force futures version to be at least 0.18

[arieroos]

I had some issues installing mqttwarn in an existing venv. This fixes #584 which is my issue.

I tested my change on my dev machine with these commands:

 pyenv install 3.10.4
 pyenv shell 3.10.4
 git clone git@github.com:arieroos/mqttwarn.git
 cd mqttwarn
 pip --version
 > pip 22.0.4 from /Users/arieroos/.pyenv/versions/3.10.4/lib/python3.10/site-packages/pip (python 3.10)
 pip install future==0.17.1
 pip install .
 mqttwarn make-config > mqttwarn.ini
 cat mqttwarn.ini

All commands succeeded

[amotl]

Thanks a stack!