Update to support python 3.6+

[BarnabasSzabolcs]

Hi,
can you please update your dependencies to support python 3.6+?
(If it is possible at all...

pip install rustress

Collecting rustress
  Downloading rustress-1.3.11.tar.gz (371 kB)
  Preparing metadata (setup.py) ... done
Collecting pymorphy2==0.8 (from rustress)
  Downloading pymorphy2-0.8-py2.py3-none-any.whl.metadata (2.3 kB)
INFO: pip is looking at multiple versions of rustress to determine which version is compatible with other requirements. This could take a while.
ERROR: Could not find a version that satisfies the requirement tensorflow==1.12.0 (from rustress) (from versions: 2.13.0rc0, 2.13.0rc1, 2.13.0rc2, 2.13.0, 2.13.1, 2.14.0rc0, 2.14.0rc1, 2.14.0, 2.14.1, 2.15.0rc0, 2.15.0rc1, 2.15.0, 2.15.1, 2.16.0rc0, 2.16.1, 2.16.2, 2.17.0rc0, 2.17.0rc1, 2.17.0, 2.17.1, 2.18.0rc0, 2.18.0rc1, 2.18.0rc2, 2.18.0)
ERROR: No matching distribution found for tensorflow==1.12.0

Thanks a lot!
Barney