feat: remove "future" dependency

[tzok]

Fixes #38

[epeisach]

This PR essentially removes python 2.7 from the this package. We need to discuss internally first about this.

However, some aspects were missed.
a) There should be no changes to to pybind11_2_6_3_dev1. This is a submodule pointing to pybind11's repository. However, if we drop python 2.7 and < 3.6 - then CMakeLists.txt should remove inclusion of this directory.

b) setup.py should be changed - Classifiers should not list python 2.7, and a "requires-python" should be added to indicate the minimal python version. This will ensure that people using python 2, with recent setup.py and pip will not try to install the newer version. See https://packaging.python.org/en/latest/guides/dropping-older-python-versions/

We need to continue to support python3.6 - as it is still the default python3 version in centos 7 --- which is still in use in numerous places. Earlier versions of python 3 I am no sure